On 04/02/2015 05:01 PM, Brian Gerst wrote:
> On Thu, Apr 2, 2015 at 10:36 AM, Denys Vlasenko <dvlas...@redhat.com> wrote:
>> stub_sigreturn ignores old values of pt_regs->REG for all general-purpose
>> registers, it sets them to values saved on userspace
>> signal stack.
>>
>> Which is hardly surprising - it would be a bug if it would use pt_regs->REG.
>> sigreturn must restore all registers.
>>
>> Therefore, SAVE_EXTRA_REGS in it ought to be redundant.
>>
>> It is a leftover from the time SAVE_EXTRA_REGS wasn't only saving registers,
>> but it also was extending stack to "full" pt_regs.
>>
>> Delete this SAVE_EXTRA_REGS.
>>
>> Run-tested.
>>
>> Signed-off-by: Denys Vlasenko <dvlas...@redhat.com>
>> CC: Linus Torvalds <torva...@linux-foundation.org>
>> CC: Steven Rostedt <rost...@goodmis.org>
>> CC: Ingo Molnar <mi...@kernel.org>
>> CC: Borislav Petkov <b...@alien8.de>
>> CC: "H. Peter Anvin" <h...@zytor.com>
>> CC: Andy Lutomirski <l...@amacapital.net>
>> CC: Oleg Nesterov <o...@redhat.com>
>> CC: Frederic Weisbecker <fweis...@gmail.com>
>> CC: Alexei Starovoitov <a...@plumgrid.com>
>> CC: Will Drewry <w...@chromium.org>
>> CC: Kees Cook <keesc...@chromium.org>
>> CC: x...@kernel.org
>> CC: linux-kernel@vger.kernel.org
>> ---
>> arch/x86/kernel/entry_64.S | 9 +++++++--
>> 1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
>> index ec51598..1cf245d 100644
>> --- a/arch/x86/kernel/entry_64.S
>> +++ b/arch/x86/kernel/entry_64.S
>> @@ -447,7 +447,12 @@ ENTRY(stub_rt_sigreturn)
>> CFI_STARTPROC
>> addq $8, %rsp
>> DEFAULT_FRAME 0
>> - SAVE_EXTRA_REGS
>> + /*
>> + * Despite RESTORE_EXTRA_REGS in return_from_stub,
>> + * no need to SAVE_EXTRA_REGS here:
>> + * sys_rt_sigreturn overwrites all general purpose pt_regs->REGs
>> + * on stack, for RESTORE_{EXTRA,C}_REGS to pick them up.
>> + */
>> call sys_rt_sigreturn
>> jmp return_from_stub
>> CFI_ENDPROC
>> @@ -458,7 +463,7 @@ ENTRY(stub_x32_rt_sigreturn)
>> CFI_STARTPROC
>> addq $8, %rsp
>> DEFAULT_FRAME 0
>> - SAVE_EXTRA_REGS
>> + /* No need to SAVE_EXTRA_REGS */
>> call sys32_x32_rt_sigreturn
>> jmp return_from_stub
>> CFI_ENDPROC
>
> I had the same idea, but determined sigreturn can fault and return an
> error code without modifying all the registers. This would leak junk
> from the stack.
This still can be made to work by not RESTORE'ing EXTRA_REGS either,
if there is a way to detect the failure:
call sys_rt_sigreturn
- jmp return_from_stub
+ testl ???????????
+ jz return_from_stub
+ ret
CFI_ENDPROC
But this is not a normal syscall, off-hand I don't see an easy way
to do the test. sys_rt_sigreturn() on failure runs this code:
...
segfault:
force_sig(SIGSEGV, current);
return 0;
}
Help?
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