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Subject[PATCH 3.2 087/104] x86/syscall: Sanitize syscall table de-references under speculation
3.2.101-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ben Hutchings <ben@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.williams@intel.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: kernel-hardening@lists.openwall.com
Cc: gregkh@linuxfoundation.org
Cc: Andy Lutomirski <luto@kernel.org>
Cc: alan@linux.intel.com
Cc: Jinpu Wang <jinpu.wang@profitbricks.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -429,6 +429,8 @@ sysenter_past_esp:
sysenter_do_call:
cmpl $(nr_syscalls), %eax
jae sysenter_badsys
+ sbb %edx, %edx /* array_index_mask_nospec() */
+ and %edx, %eax
call *sys_call_table(,%eax,4)
sysenter_after_call:
movl %eax,PT_EAX(%esp)
@@ -512,6 +514,8 @@ ENTRY(system_call)
cmpl $(nr_syscalls), %eax
jae syscall_badsys
syscall_call:
+ sbb %edx, %edx /* array_index_mask_nospec() */
+ and %edx, %eax
call *sys_call_table(,%eax,4)
syscall_after_call:
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
system_call_fastpath:
- 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
#ifdef CONFIG_RETPOLINE
movq sys_call_table(, %rax, 8), %rax
@@ -646,8 +648,10 @@ tracesys:
*/
LOAD_ARGS ARGOFFSET, 1
RESTORE_REST
- 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 */
#ifdef CONFIG_RETPOLINE
movq sys_call_table(, %rax, 8), %rax
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