Re: [PATCH v2] Btrfs: fix out of bounds array access while reading extent buffer

[Liu Bo]

On Wed, Aug 09, 2017 at 06:40:04PM +0100, Filipe Manana wrote:
> On Wed, Aug 9, 2017 at 5:31 PM, Liu Bo  wrote:
> > There is a cornel case that slip through the checkers in functions
> > reading extent buffer, ie.
> >
> > if (start < eb->len) and (start + len > eb->len),
> > then
> >
> > a) map_private_extent_buffer() returns immediately because
> > it's thinking the range spans across two pages,
> >
> > b) and the checkers in read_extent_buffer(), WARN_ON(start > eb->len)
> > and WARN_ON(start + len > eb->start + eb->len), both are OK in this
> > corner case, but it'd actually try to access the eb->pages out of
> > bounds because of (start + len > eb->len).
> >
> > The case is found by switching extent inline ref type from shared data
> > ref to non-shared data ref, which is a kind of metadata corruption.
> >
> > It'd use the wrong helper to access the eb,
> > eg. btrfs_extent_data_ref_root(eb, ref) is used but the %ref passing
> > here is "struct btrfs_shared_data_ref".  And if the extent item
> > happens to be the first item in the eb, then offset/length will get
> > over eb->len which ends up an invalid memory access.
> >
> > This is adding proper checks in order to avoid invalid memory access,
> > ie. 'general protection fault', before it's too late.
> >
> > Signed-off-by: Liu Bo 
> > ---
> >
> > v2: Improve the commit log to clarify that this can only happen if
> > metadata is corrupted.
> >
> >  fs/btrfs/extent_io.c | 22 ++
> >  1 file changed, 14 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> > index 0aff9b2..d198e87 100644
> > --- a/fs/btrfs/extent_io.c
> > +++ b/fs/btrfs/extent_io.c
> > @@ -5416,13 +5416,19 @@ void read_extent_buffer(struct extent_buffer *eb, 
> > void *dstv,
> > char *dst = (char *)dstv;
> > size_t start_offset = eb->start & ((u64)PAGE_SIZE - 1);
> > unsigned long i = (start_offset + start) >> PAGE_SHIFT;
> > +   unsigned long num_pages = num_extent_pages(eb->start, eb->len);
> 
> Forgot to note that if CONFIG_BTRFS_ASSERT is not set, this variable
> is unused and the compiler and static checkers will emit a warning.
> Other than that all looks fine. Thanks.
>

True, then lets remove it, I don't think we could reach that ASSERT if
(start + len > eb->len).

thanks,

-liubo

> >
> > -   WARN_ON(start > eb->len);
> > -   WARN_ON(start + len > eb->start + eb->len);
> > +   if (start + len > eb->len) {
> > +   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
> > wanted %lu %lu\n",
> > +eb->start, eb->len, start, len);
> > +   memset(dst, 0, len);
> > +   return;
> > +   }
> >
> > offset = (start_offset + start) & (PAGE_SIZE - 1);
> >
> > while (len > 0) {
> > +   ASSERT(i < num_pages);
> > page = eb->pages[i];
> >
> > cur = min(len, (PAGE_SIZE - offset));
> > @@ -5491,6 +5497,12 @@ int map_private_extent_buffer(struct extent_buffer 
> > *eb, unsigned long start,
> > unsigned long end_i = (start_offset + start + min_len - 1) >>
> > PAGE_SHIFT;
> >
> > +   if (start + min_len > eb->len) {
> > +   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
> > wanted %lu %lu\n",
> > +  eb->start, eb->len, start, min_len);
> > +   return -EINVAL;
> > +   }
> > +
> > if (i != end_i)
> > return 1;
> >
> > @@ -5502,12 +5514,6 @@ int map_private_extent_buffer(struct extent_buffer 
> > *eb, unsigned long start,
> > *map_start = ((u64)i << PAGE_SHIFT) - start_offset;
> > }
> >
> > -   if (start + min_len > eb->len) {
> > -   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
> > wanted %lu %lu\n",
> > -  eb->start, eb->len, start, min_len);
> > -   return -EINVAL;
> > -   }
> > -
> > p = eb->pages[i];
> > kaddr = page_address(p);
> > *map = kaddr + offset;
> > --
> > 2.9.4
> >
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> > the body of a message to majord...@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> 
> 
> -- 
> Filipe David Manana,
> 
> “Whether you think you can, or you think you can't — you're right.”
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majord...@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
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Re: [PATCH v2] Btrfs: fix out of bounds array access while reading extent buffer

[Filipe Manana]

On Wed, Aug 9, 2017 at 5:31 PM, Liu Bo  wrote:
> There is a cornel case that slip through the checkers in functions
> reading extent buffer, ie.
>
> if (start < eb->len) and (start + len > eb->len),
> then
>
> a) map_private_extent_buffer() returns immediately because
> it's thinking the range spans across two pages,
>
> b) and the checkers in read_extent_buffer(), WARN_ON(start > eb->len)
> and WARN_ON(start + len > eb->start + eb->len), both are OK in this
> corner case, but it'd actually try to access the eb->pages out of
> bounds because of (start + len > eb->len).
>
> The case is found by switching extent inline ref type from shared data
> ref to non-shared data ref, which is a kind of metadata corruption.
>
> It'd use the wrong helper to access the eb,
> eg. btrfs_extent_data_ref_root(eb, ref) is used but the %ref passing
> here is "struct btrfs_shared_data_ref".  And if the extent item
> happens to be the first item in the eb, then offset/length will get
> over eb->len which ends up an invalid memory access.
>
> This is adding proper checks in order to avoid invalid memory access,
> ie. 'general protection fault', before it's too late.
>
> Signed-off-by: Liu Bo 
> ---
>
> v2: Improve the commit log to clarify that this can only happen if
> metadata is corrupted.
>
>  fs/btrfs/extent_io.c | 22 ++
>  1 file changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 0aff9b2..d198e87 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -5416,13 +5416,19 @@ void read_extent_buffer(struct extent_buffer *eb, 
> void *dstv,
> char *dst = (char *)dstv;
> size_t start_offset = eb->start & ((u64)PAGE_SIZE - 1);
> unsigned long i = (start_offset + start) >> PAGE_SHIFT;
> +   unsigned long num_pages = num_extent_pages(eb->start, eb->len);

Forgot to note that if CONFIG_BTRFS_ASSERT is not set, this variable
is unused and the compiler and static checkers will emit a warning.
Other than that all looks fine. Thanks.

>
> -   WARN_ON(start > eb->len);
> -   WARN_ON(start + len > eb->start + eb->len);
> +   if (start + len > eb->len) {
> +   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
> wanted %lu %lu\n",
> +eb->start, eb->len, start, len);
> +   memset(dst, 0, len);
> +   return;
> +   }
>
> offset = (start_offset + start) & (PAGE_SIZE - 1);
>
> while (len > 0) {
> +   ASSERT(i < num_pages);
> page = eb->pages[i];
>
> cur = min(len, (PAGE_SIZE - offset));
> @@ -5491,6 +5497,12 @@ int map_private_extent_buffer(struct extent_buffer 
> *eb, unsigned long start,
> unsigned long end_i = (start_offset + start + min_len - 1) >>
> PAGE_SHIFT;
>
> +   if (start + min_len > eb->len) {
> +   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
> wanted %lu %lu\n",
> +  eb->start, eb->len, start, min_len);
> +   return -EINVAL;
> +   }
> +
> if (i != end_i)
> return 1;
>
> @@ -5502,12 +5514,6 @@ int map_private_extent_buffer(struct extent_buffer 
> *eb, unsigned long start,
> *map_start = ((u64)i << PAGE_SHIFT) - start_offset;
> }
>
> -   if (start + min_len > eb->len) {
> -   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
> wanted %lu %lu\n",
> -  eb->start, eb->len, start, min_len);
> -   return -EINVAL;
> -   }
> -
> p = eb->pages[i];
> kaddr = page_address(p);
> *map = kaddr + offset;
> --
> 2.9.4
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majord...@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html



-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”
--
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Re: [PATCH v2] Btrfs: fix out of bounds array access while reading extent buffer

[Filipe Manana]

On Wed, Aug 9, 2017 at 5:31 PM, Liu Bo  wrote:
> There is a cornel case that slip through the checkers in functions
> reading extent buffer, ie.
>
> if (start < eb->len) and (start + len > eb->len),
> then
>
> a) map_private_extent_buffer() returns immediately because
> it's thinking the range spans across two pages,
>
> b) and the checkers in read_extent_buffer(), WARN_ON(start > eb->len)
> and WARN_ON(start + len > eb->start + eb->len), both are OK in this
> corner case, but it'd actually try to access the eb->pages out of
> bounds because of (start + len > eb->len).
>
> The case is found by switching extent inline ref type from shared data
> ref to non-shared data ref, which is a kind of metadata corruption.
>
> It'd use the wrong helper to access the eb,
> eg. btrfs_extent_data_ref_root(eb, ref) is used but the %ref passing
> here is "struct btrfs_shared_data_ref".  And if the extent item
> happens to be the first item in the eb, then offset/length will get
> over eb->len which ends up an invalid memory access.
>
> This is adding proper checks in order to avoid invalid memory access,
> ie. 'general protection fault', before it's too late.
>
> Signed-off-by: Liu Bo 
Reviewed-by: Filipe Manana 

> ---
>
> v2: Improve the commit log to clarify that this can only happen if
> metadata is corrupted.

Thanks for adding the clarification!

>
>  fs/btrfs/extent_io.c | 22 ++
>  1 file changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 0aff9b2..d198e87 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -5416,13 +5416,19 @@ void read_extent_buffer(struct extent_buffer *eb, 
> void *dstv,
> char *dst = (char *)dstv;
> size_t start_offset = eb->start & ((u64)PAGE_SIZE - 1);
> unsigned long i = (start_offset + start) >> PAGE_SHIFT;
> +   unsigned long num_pages = num_extent_pages(eb->start, eb->len);
>
> -   WARN_ON(start > eb->len);
> -   WARN_ON(start + len > eb->start + eb->len);
> +   if (start + len > eb->len) {
> +   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
> wanted %lu %lu\n",
> +eb->start, eb->len, start, len);
> +   memset(dst, 0, len);
> +   return;
> +   }
>
> offset = (start_offset + start) & (PAGE_SIZE - 1);
>
> while (len > 0) {
> +   ASSERT(i < num_pages);
> page = eb->pages[i];
>
> cur = min(len, (PAGE_SIZE - offset));
> @@ -5491,6 +5497,12 @@ int map_private_extent_buffer(struct extent_buffer 
> *eb, unsigned long start,
> unsigned long end_i = (start_offset + start + min_len - 1) >>
> PAGE_SHIFT;
>
> +   if (start + min_len > eb->len) {
> +   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
> wanted %lu %lu\n",
> +  eb->start, eb->len, start, min_len);
> +   return -EINVAL;
> +   }
> +
> if (i != end_i)
> return 1;
>
> @@ -5502,12 +5514,6 @@ int map_private_extent_buffer(struct extent_buffer 
> *eb, unsigned long start,
> *map_start = ((u64)i << PAGE_SHIFT) - start_offset;
> }
>
> -   if (start + min_len > eb->len) {
> -   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
> wanted %lu %lu\n",
> -  eb->start, eb->len, start, min_len);
> -   return -EINVAL;
> -   }
> -
> p = eb->pages[i];
> kaddr = page_address(p);
> *map = kaddr + offset;
> --
> 2.9.4
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majord...@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html



-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
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[PATCH v2] Btrfs: fix out of bounds array access while reading extent buffer

[Liu Bo]

There is a cornel case that slip through the checkers in functions
reading extent buffer, ie.

if (start < eb->len) and (start + len > eb->len),
then

a) map_private_extent_buffer() returns immediately because
it's thinking the range spans across two pages,

b) and the checkers in read_extent_buffer(), WARN_ON(start > eb->len)
and WARN_ON(start + len > eb->start + eb->len), both are OK in this
corner case, but it'd actually try to access the eb->pages out of
bounds because of (start + len > eb->len).

The case is found by switching extent inline ref type from shared data
ref to non-shared data ref, which is a kind of metadata corruption.

It'd use the wrong helper to access the eb,
eg. btrfs_extent_data_ref_root(eb, ref) is used but the %ref passing
here is "struct btrfs_shared_data_ref".  And if the extent item
happens to be the first item in the eb, then offset/length will get
over eb->len which ends up an invalid memory access.

This is adding proper checks in order to avoid invalid memory access,
ie. 'general protection fault', before it's too late.

Signed-off-by: Liu Bo 
---

v2: Improve the commit log to clarify that this can only happen if
metadata is corrupted.

 fs/btrfs/extent_io.c | 22 ++
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 0aff9b2..d198e87 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -5416,13 +5416,19 @@ void read_extent_buffer(struct extent_buffer *eb, void 
*dstv,
char *dst = (char *)dstv;
size_t start_offset = eb->start & ((u64)PAGE_SIZE - 1);
unsigned long i = (start_offset + start) >> PAGE_SHIFT;
+   unsigned long num_pages = num_extent_pages(eb->start, eb->len);
 
-   WARN_ON(start > eb->len);
-   WARN_ON(start + len > eb->start + eb->len);
+   if (start + len > eb->len) {
+   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
wanted %lu %lu\n",
+eb->start, eb->len, start, len);
+   memset(dst, 0, len);
+   return;
+   }
 
offset = (start_offset + start) & (PAGE_SIZE - 1);
 
while (len > 0) {
+   ASSERT(i < num_pages);
page = eb->pages[i];
 
cur = min(len, (PAGE_SIZE - offset));
@@ -5491,6 +5497,12 @@ int map_private_extent_buffer(struct extent_buffer *eb, 
unsigned long start,
unsigned long end_i = (start_offset + start + min_len - 1) >>
PAGE_SHIFT;
 
+   if (start + min_len > eb->len) {
+   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
wanted %lu %lu\n",
+  eb->start, eb->len, start, min_len);
+   return -EINVAL;
+   }
+
if (i != end_i)
return 1;
 
@@ -5502,12 +5514,6 @@ int map_private_extent_buffer(struct extent_buffer *eb, 
unsigned long start,
*map_start = ((u64)i << PAGE_SHIFT) - start_offset;
}
 
-   if (start + min_len > eb->len) {
-   WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
wanted %lu %lu\n",
-  eb->start, eb->len, start, min_len);
-   return -EINVAL;
-   }
-
p = eb->pages[i];
kaddr = page_address(p);
*map = kaddr + offset;
-- 
2.9.4

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