3.2.101-rc1 review patch.  If anyone has any objections, please let me know.


From: Ben Hutchings <b...@decadent.org.uk>

commit 2fbd7af5af8665d18bcefae3e9700be07e22b681 upstream.

The upstream version of this, touching C code, was written by Dan Williams,
with the following description:

> The syscall table base is a user controlled function pointer in kernel
> space. Use array_index_nospec() to prevent any out of bounds speculation.
> While retpoline prevents speculating into a userspace directed target it
> does not stop the pointer de-reference, the concern is leaking memory
> relative to the syscall table base, by observing instruction cache
> behavior.

The x86_64 assembly version for 4.4 was written by Jiri Slaby, with
the following description:

> In 4.4.118, we have commit c8961332d6da (x86/syscall: Sanitize syscall
> table de-references under speculation), which is a backport of upstream
> commit 2fbd7af5af86. But it fixed only the C part of the upstream patch
> -- the IA32 sysentry. So it ommitted completely the assembly part -- the
> 64bit sysentry.
> Fix that in this patch by explicit array_index_mask_nospec written in
> assembly. The same was used in lib/getuser.S.
> However, to have "sbb" working properly, we have to switch from "cmp"
> against (NR_syscalls-1) to (NR_syscalls), otherwise the last syscall
> number would be "and"ed by 0. It is because the original "ja" relies on
> "CF" or "ZF", but we rely only on "CF" in "sbb". That means: switch to
> "jae" conditional jump too.
> Final note: use rcx for mask as this is exactly what is overwritten by
> the 4th syscall argument (r10) right after.

In 3.2 the x86_32 syscall table lookup is also written in assembly.
So I've taken Jiri's version and added similar masking in entry_32.S,
using edx as the temporary.  edx is clobbered by SAVE_REGS and seems
to be free at this point.

In 3.2 the x86_64 entry code also lacks syscall masking for x32.

Cc: Dan Williams <dan.j.willi...@intel.com>
Cc: Jiri Slaby <jsl...@suse.cz>
Cc: Jan Beulich <jbeul...@suse.com>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: linux-a...@vger.kernel.org
Cc: kernel-harden...@lists.openwall.com
Cc: gre...@linuxfoundation.org
Cc: Andy Lutomirski <l...@kernel.org>
Cc: a...@linux.intel.com
Cc: Jinpu Wang <jinpu.w...@profitbricks.com>
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -429,6 +429,8 @@ sysenter_past_esp:
        cmpl $(nr_syscalls), %eax
        jae sysenter_badsys
+       sbb %edx, %edx                          /* array_index_mask_nospec() */
+       and %edx, %eax
        call *sys_call_table(,%eax,4)
        movl %eax,PT_EAX(%esp)
@@ -512,6 +514,8 @@ ENTRY(system_call)
        cmpl $(nr_syscalls), %eax
        jae syscall_badsys
+       sbb %edx, %edx                          /* array_index_mask_nospec() */
+       and %edx, %eax
        call *sys_call_table(,%eax,4)
        movl %eax,PT_EAX(%esp)          # store the return value
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -517,8 +517,10 @@ ENTRY(system_call_after_swapgs)
        testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags(%rcx)
        jnz tracesys
-       cmpq $__NR_syscall_max,%rax
-       ja badsys
+       cmpq    $NR_syscalls, %rax
+       jae     badsys
+       sbb     %rcx, %rcx                      /* array_index_mask_nospec() */
+       and     %rcx, %rax
        movq %r10,%rcx
        movq    sys_call_table(, %rax, 8), %rax
@@ -646,8 +648,10 @@ tracesys:
-       cmpq $__NR_syscall_max,%rax
-       ja   int_ret_from_sys_call      /* RAX(%rsp) set to -ENOSYS above */
+       cmpq    $NR_syscalls, %rax
+       jae     int_ret_from_sys_call           /* RAX(%rsp) set to -ENOSYS 
above */
+       sbb     %rcx, %rcx                      /* array_index_mask_nospec() */
+       and     %rcx, %rax
        movq %r10,%rcx  /* fixup for C */
        movq    sys_call_table(, %rax, 8), %rax

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