Re: [PATCH 4.4 178/193] x86/syscall: Sanitize syscall table de-references under speculation

[Jiri Slaby]

On 03/06/2018, 03:21 PM, Jiri Slaby wrote:
> On 02/23/2018, 07:26 PM, Greg Kroah-Hartman wrote:
>> 4.4-stable review patch.  If anyone has any objections, please let me know.
>>
>> ------------------
>>
>> From: Dan Williams <dan.j.williams@xxxxxxxxx>
>>
>> (cherry picked from commit 2fbd7af5af8665d18bcefae3e9700be07e22b681)
>>
>> 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.
>>
>> Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: linux-arch@xxxxxxxxxxxxxxx
>> Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
>> Cc: gregkh@xxxxxxxxxxxxxxxxxxx
>> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
>> Cc: alan@xxxxxxxxxxxxxxx
>> Link: https://lkml.kernel.org/r/151727417984.33451.1216731042505722161.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
>> [jwang: port to 4.4, no syscall_64]
> 
> This is not complete IMO, the syscall is indeed there, only written in
> assembly in 4.4 yet.
> 
> So this patch looks like it is missing these two hunks (from my
> SLE12-SP2 backport):
> 
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -184,6 +184,8 @@ entry_SYSCALL_64_fastpath:
>>        cmpl    $__NR_syscall_max, %eax
>>  #endif
>>        ja      1f                              /* return -ENOSYS (already in pt_regs->ax) */
>> +      sbb     %rcx, %rcx                      /* array_index_mask_nospec() */
>> +      and     %rcx, %rax

Which is not completely correct either. The preceding comparison should
write:
    cmpl    $NR_syscalls, %eax
    jae     1f
to have sbb correctly working even on the last syscall number.

thanks,
-- 
js
suse labs