Re: [PATCH v2 1/4] powerpc/64s: implement probe_kernel_read/write without touching AMR
Excerpts from Christophe Leroy's message of June 10, 2020 10:41 pm: > Hi Nick > > Le 03/04/2020 à 11:35, Nicholas Piggin a écrit : >> There is no need to allow user accesses when probing kernel addresses. > > You should have a look at > https://github.com/torvalds/linux/commit/fa94111d94354de76c47fea6e1187d1ee91e23a7 > > At seems to implement a generic way of achieving what you are trying to > do here. Yep thanks for that, I'll rebase this series on upstream now. Thanks, Nick
Re: [PATCH v2 1/4] powerpc/64s: implement probe_kernel_read/write without touching AMR
Hi Nick
Le 03/04/2020 à 11:35, Nicholas Piggin a écrit :
There is no need to allow user accesses when probing kernel addresses.
You should have a look at
https://github.com/torvalds/linux/commit/fa94111d94354de76c47fea6e1187d1ee91e23a7
At seems to implement a generic way of achieving what you are trying to
do here.
Christophe
Signed-off-by: Nicholas Piggin
---
v2:
- Enable for all powerpc (suggested by Christophe)
- Fold helper function together (Christophe)
- Rename uaccess.c to maccess.c to match kernel/maccess.c.
arch/powerpc/include/asm/uaccess.h | 25 +++---
arch/powerpc/lib/Makefile | 2 +-
arch/powerpc/lib/maccess.c | 34 ++
3 files changed, 52 insertions(+), 9 deletions(-)
create mode 100644 arch/powerpc/lib/maccess.c
diff --git a/arch/powerpc/include/asm/uaccess.h
b/arch/powerpc/include/asm/uaccess.h
index 2f500debae21..670910df3cc7 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -341,8 +341,8 @@ raw_copy_in_user(void __user *to, const void __user *from,
unsigned long n)
}
#endif /* __powerpc64__ */
-static inline unsigned long raw_copy_from_user(void *to,
- const void __user *from, unsigned long n)
+static inline unsigned long
+raw_copy_from_user_allowed(void *to, const void __user *from, unsigned long n)
{
unsigned long ret;
if (__builtin_constant_p(n) && (n <= 8)) {
@@ -351,19 +351,19 @@ static inline unsigned long raw_copy_from_user(void *to,
switch (n) {
case 1:
barrier_nospec();
- __get_user_size(*(u8 *)to, from, 1, ret);
+ __get_user_size_allowed(*(u8 *)to, from, 1, ret);
break;
case 2:
barrier_nospec();
- __get_user_size(*(u16 *)to, from, 2, ret);
+ __get_user_size_allowed(*(u16 *)to, from, 2, ret);
break;
case 4:
barrier_nospec();
- __get_user_size(*(u32 *)to, from, 4, ret);
+ __get_user_size_allowed(*(u32 *)to, from, 4, ret);
break;
case 8:
barrier_nospec();
- __get_user_size(*(u64 *)to, from, 8, ret);
+ __get_user_size_allowed(*(u64 *)to, from, 8, ret);
break;
}
if (ret == 0)
@@ -371,9 +371,18 @@ static inline unsigned long raw_copy_from_user(void *to,
}
barrier_nospec();
- allow_read_from_user(from, n);
ret = __copy_tofrom_user((__force void __user *)to, from, n);
- prevent_read_from_user(from, n);
+ return ret;
+}
+
+static inline unsigned long
+raw_copy_from_user(void *to, const void __user *from, unsigned long n)
+{
+ unsigned long ret;
+
+ allow_read_from_user(to, n);
+ ret = raw_copy_from_user_allowed(to, from, n);
+ prevent_read_from_user(to, n);
return ret;
}
diff --git a/arch/powerpc/lib/Makefile b/arch/powerpc/lib/Makefile
index b8de3be10eb4..77af10ad0b0d 100644
--- a/arch/powerpc/lib/Makefile
+++ b/arch/powerpc/lib/Makefile
@@ -16,7 +16,7 @@ CFLAGS_code-patching.o += -DDISABLE_BRANCH_PROFILING
CFLAGS_feature-fixups.o += -DDISABLE_BRANCH_PROFILING
endif
-obj-y += alloc.o code-patching.o feature-fixups.o pmem.o
+obj-y += alloc.o code-patching.o feature-fixups.o pmem.o maccess.o
ifndef CONFIG_KASAN
obj-y += string.o memcmp_$(BITS).o
diff --git a/arch/powerpc/lib/maccess.c b/arch/powerpc/lib/maccess.c
new file mode 100644
index ..ce5465db1e2d
--- /dev/null
+++ b/arch/powerpc/lib/maccess.c
@@ -0,0 +1,34 @@
+#include
+#include
+
+/*
+ * Override the generic weak linkage functions to avoid changing KUP state via
+ * the generic user access functions, as this is accessing kernel addresses.
+ */
+long probe_kernel_read(void *dst, const void *src, size_t size)
+{
+ long ret;
+ mm_segment_t old_fs = get_fs();
+
+ set_fs(KERNEL_DS);
+ pagefault_disable();
+ ret = raw_copy_from_user_allowed(dst, (__force const void __user *)src,
size);
+ pagefault_enable();
+ set_fs(old_fs);
+
+ return ret ? -EFAULT : 0;
+}
+
+long probe_kernel_write(void *dst, const void *src, size_t size)
+{
+ long ret;
+ mm_segment_t old_fs = get_fs();
+
+ set_fs(KERNEL_DS);
+ pagefault_disable();
+ ret = raw_copy_to_user_allowed((__force void __user *)dst, src, size);
+ pagefault_enable();
+ set_fs(old_fs);
+
+ return ret ? -EFAULT : 0;
+}
Re: [PATCH v2 1/4] powerpc/64s: implement probe_kernel_read/write without touching AMR
Hey Nicholas, On 4/7/20 6:01 AM, Nicholas Piggin wrote: Nicholas Piggin's on April 3, 2020 9:05 pm: Christophe Leroy's on April 3, 2020 8:31 pm: Le 03/04/2020 à 11:35, Nicholas Piggin a écrit : There is no need to allow user accesses when probing kernel addresses. I just discovered the following commit https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=75a1a607bb7e6d918be3aca11ec2214a275392f4 This commit adds probe_kernel_read_strict() and probe_kernel_write_strict(). When reading the commit log, I understand that probe_kernel_read() may be used to access some user memory. Which will not work anymore with your patch. Hmm, I looked at _strict but obviously not hard enough. Good catch. I don't think probe_kernel_read() should ever access user memory, the comment certainly says it doesn't, but that patch sort of implies that they do. I think it's wrong. The non-_strict maybe could return userspace data to you if you did pass a user address? I don't see why that shouldn't just be disallowed always though. And if the _strict version is required to be safe, then it seems like a bug or security issue to just allow everyone that doesn't explicitly override it to use the default implementation. Also, the way the weak linkage is done in that patch, means parisc and um archs that were previously overriding probe_kernel_read() now get the default probe_kernel_read_strict(), which would be wrong for them. The changelog in commit 75a1a607bb7 makes it a bit clearer. If the non-_strict variant is used on non-kernel addresses, then it might not return -EFAULT or it might cause a kernel warning. The _strict variant is supposed to be usable with any address and it will return -EFAULT if it was not a valid and mapped kernel address. The non-_strict variant can not portably access user memory because it uses KERNEL_DS, and its documentation says its only for kernel pointers. So powerpc should be fine to run that under KUAP AFAIKS. I don't know why the _strict behaviour is not just made default, but the implementation of it does seem to be broken on the archs that override the non-_strict variant. Yeah, we should make it default and only add a "opt out" for the old legacy cases; there was also same discussion started over here just recently [0]. Thanks, Daniel [0] https://lore.kernel.org/lkml/20200403133533.ga3...@infradead.org/T/
Re: [PATCH v2 1/4] powerpc/64s: implement probe_kernel_read/write without touching AMR
Nicholas Piggin's on April 3, 2020 9:05 pm: > Christophe Leroy's on April 3, 2020 8:31 pm: >> >> >> Le 03/04/2020 à 11:35, Nicholas Piggin a écrit : >>> There is no need to allow user accesses when probing kernel addresses. >> >> I just discovered the following commit >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=75a1a607bb7e6d918be3aca11ec2214a275392f4 >> >> This commit adds probe_kernel_read_strict() and probe_kernel_write_strict(). >> >> When reading the commit log, I understand that probe_kernel_read() may >> be used to access some user memory. Which will not work anymore with >> your patch. > > Hmm, I looked at _strict but obviously not hard enough. Good catch. > > I don't think probe_kernel_read() should ever access user memory, > the comment certainly says it doesn't, but that patch sort of implies > that they do. > > I think it's wrong. The non-_strict maybe could return userspace data to > you if you did pass a user address? I don't see why that shouldn't just > be disallowed always though. > > And if the _strict version is required to be safe, then it seems like a > bug or security issue to just allow everyone that doesn't explicitly > override it to use the default implementation. > > Also, the way the weak linkage is done in that patch, means parisc and > um archs that were previously overriding probe_kernel_read() now get > the default probe_kernel_read_strict(), which would be wrong for them. The changelog in commit 75a1a607bb7 makes it a bit clearer. If the non-_strict variant is used on non-kernel addresses, then it might not return -EFAULT or it might cause a kernel warning. The _strict variant is supposed to be usable with any address and it will return -EFAULT if it was not a valid and mapped kernel address. The non-_strict variant can not portably access user memory because it uses KERNEL_DS, and its documentation says its only for kernel pointers. So powerpc should be fine to run that under KUAP AFAIKS. I don't know why the _strict behaviour is not just made default, but the implementation of it does seem to be broken on the archs that override the non-_strict variant. Thanks, Nick
Re: [PATCH v2 1/4] powerpc/64s: implement probe_kernel_read/write without touching AMR
Christophe Leroy's on April 3, 2020 8:31 pm: > > > Le 03/04/2020 à 11:35, Nicholas Piggin a écrit : >> There is no need to allow user accesses when probing kernel addresses. > > I just discovered the following commit > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=75a1a607bb7e6d918be3aca11ec2214a275392f4 > > This commit adds probe_kernel_read_strict() and probe_kernel_write_strict(). > > When reading the commit log, I understand that probe_kernel_read() may > be used to access some user memory. Which will not work anymore with > your patch. Hmm, I looked at _strict but obviously not hard enough. Good catch. I don't think probe_kernel_read() should ever access user memory, the comment certainly says it doesn't, but that patch sort of implies that they do. I think it's wrong. The non-_strict maybe could return userspace data to you if you did pass a user address? I don't see why that shouldn't just be disallowed always though. And if the _strict version is required to be safe, then it seems like a bug or security issue to just allow everyone that doesn't explicitly override it to use the default implementation. Also, the way the weak linkage is done in that patch, means parisc and um archs that were previously overriding probe_kernel_read() now get the default probe_kernel_read_strict(), which would be wrong for them. > > Isn't it probe_kernel_read_strict() and probe_kernel_write_strict() that > you want to add ? > >> >> Signed-off-by: Nicholas Piggin >> --- >> v2: >> - Enable for all powerpc (suggested by Christophe) >> - Fold helper function together (Christophe) >> - Rename uaccess.c to maccess.c to match kernel/maccess.c. >> >> arch/powerpc/include/asm/uaccess.h | 25 +++--- >> arch/powerpc/lib/Makefile | 2 +- >> arch/powerpc/lib/maccess.c | 34 ++ > > x86 does it in mm/maccess.c Yeah I'll fix that up, thanks. Thanks, Nick
Re: [PATCH v2 1/4] powerpc/64s: implement probe_kernel_read/write without touching AMR
Le 03/04/2020 à 11:35, Nicholas Piggin a écrit : There is no need to allow user accesses when probing kernel addresses. I just discovered the following commit https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=75a1a607bb7e6d918be3aca11ec2214a275392f4 This commit adds probe_kernel_read_strict() and probe_kernel_write_strict(). When reading the commit log, I understand that probe_kernel_read() may be used to access some user memory. Which will not work anymore with your patch. Isn't it probe_kernel_read_strict() and probe_kernel_write_strict() that you want to add ? Signed-off-by: Nicholas Piggin --- v2: - Enable for all powerpc (suggested by Christophe) - Fold helper function together (Christophe) - Rename uaccess.c to maccess.c to match kernel/maccess.c. arch/powerpc/include/asm/uaccess.h | 25 +++--- arch/powerpc/lib/Makefile | 2 +- arch/powerpc/lib/maccess.c | 34 ++ x86 does it in mm/maccess.c 3 files changed, 52 insertions(+), 9 deletions(-) create mode 100644 arch/powerpc/lib/maccess.c Christophe
[PATCH v2 1/4] powerpc/64s: implement probe_kernel_read/write without touching AMR
There is no need to allow user accesses when probing kernel addresses.
Signed-off-by: Nicholas Piggin
---
v2:
- Enable for all powerpc (suggested by Christophe)
- Fold helper function together (Christophe)
- Rename uaccess.c to maccess.c to match kernel/maccess.c.
arch/powerpc/include/asm/uaccess.h | 25 +++---
arch/powerpc/lib/Makefile | 2 +-
arch/powerpc/lib/maccess.c | 34 ++
3 files changed, 52 insertions(+), 9 deletions(-)
create mode 100644 arch/powerpc/lib/maccess.c
diff --git a/arch/powerpc/include/asm/uaccess.h
b/arch/powerpc/include/asm/uaccess.h
index 2f500debae21..670910df3cc7 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -341,8 +341,8 @@ raw_copy_in_user(void __user *to, const void __user *from,
unsigned long n)
}
#endif /* __powerpc64__ */
-static inline unsigned long raw_copy_from_user(void *to,
- const void __user *from, unsigned long n)
+static inline unsigned long
+raw_copy_from_user_allowed(void *to, const void __user *from, unsigned long n)
{
unsigned long ret;
if (__builtin_constant_p(n) && (n <= 8)) {
@@ -351,19 +351,19 @@ static inline unsigned long raw_copy_from_user(void *to,
switch (n) {
case 1:
barrier_nospec();
- __get_user_size(*(u8 *)to, from, 1, ret);
+ __get_user_size_allowed(*(u8 *)to, from, 1, ret);
break;
case 2:
barrier_nospec();
- __get_user_size(*(u16 *)to, from, 2, ret);
+ __get_user_size_allowed(*(u16 *)to, from, 2, ret);
break;
case 4:
barrier_nospec();
- __get_user_size(*(u32 *)to, from, 4, ret);
+ __get_user_size_allowed(*(u32 *)to, from, 4, ret);
break;
case 8:
barrier_nospec();
- __get_user_size(*(u64 *)to, from, 8, ret);
+ __get_user_size_allowed(*(u64 *)to, from, 8, ret);
break;
}
if (ret == 0)
@@ -371,9 +371,18 @@ static inline unsigned long raw_copy_from_user(void *to,
}
barrier_nospec();
- allow_read_from_user(from, n);
ret = __copy_tofrom_user((__force void __user *)to, from, n);
- prevent_read_from_user(from, n);
+ return ret;
+}
+
+static inline unsigned long
+raw_copy_from_user(void *to, const void __user *from, unsigned long n)
+{
+ unsigned long ret;
+
+ allow_read_from_user(to, n);
+ ret = raw_copy_from_user_allowed(to, from, n);
+ prevent_read_from_user(to, n);
return ret;
}
diff --git a/arch/powerpc/lib/Makefile b/arch/powerpc/lib/Makefile
index b8de3be10eb4..77af10ad0b0d 100644
--- a/arch/powerpc/lib/Makefile
+++ b/arch/powerpc/lib/Makefile
@@ -16,7 +16,7 @@ CFLAGS_code-patching.o += -DDISABLE_BRANCH_PROFILING
CFLAGS_feature-fixups.o += -DDISABLE_BRANCH_PROFILING
endif
-obj-y += alloc.o code-patching.o feature-fixups.o pmem.o
+obj-y += alloc.o code-patching.o feature-fixups.o pmem.o maccess.o
ifndef CONFIG_KASAN
obj-y += string.o memcmp_$(BITS).o
diff --git a/arch/powerpc/lib/maccess.c b/arch/powerpc/lib/maccess.c
new file mode 100644
index ..ce5465db1e2d
--- /dev/null
+++ b/arch/powerpc/lib/maccess.c
@@ -0,0 +1,34 @@
+#include
+#include
+
+/*
+ * Override the generic weak linkage functions to avoid changing KUP state via
+ * the generic user access functions, as this is accessing kernel addresses.
+ */
+long probe_kernel_read(void *dst, const void *src, size_t size)
+{
+ long ret;
+ mm_segment_t old_fs = get_fs();
+
+ set_fs(KERNEL_DS);
+ pagefault_disable();
+ ret = raw_copy_from_user_allowed(dst, (__force const void __user *)src,
size);
+ pagefault_enable();
+ set_fs(old_fs);
+
+ return ret ? -EFAULT : 0;
+}
+
+long probe_kernel_write(void *dst, const void *src, size_t size)
+{
+ long ret;
+ mm_segment_t old_fs = get_fs();
+
+ set_fs(KERNEL_DS);
+ pagefault_disable();
+ ret = raw_copy_to_user_allowed((__force void __user *)dst, src, size);
+ pagefault_enable();
+ set_fs(old_fs);
+
+ return ret ? -EFAULT : 0;
+}
--
2.23.0
