Re: [Qemu-devel] [PATCH 2/2] target/i386: trap on instructions longer than >15 bytes

[Paolo Bonzini <pbonzini@redhat.com>]

On 12/10/2017 21:30, Richard Henderson wrote:
> On 10/12/2017 07:35 AM, Paolo Bonzini wrote:
>> Besides being more correct, arbitrarily long instruction allow the
>> generation of a translation block that spans three pages.  This
>> confuses the generator and even allows ring 3 code to poison the
>> translation block cache and inject code into other processes that are
>> in guest ring 3.
>>
>> This is an improved (and more invasive) fix for the bug fixed in commit
>> 30663fd ("tcg/i386: Check the size of instruction being translated",
>> 2017-03-24).  In addition to being more precise (and generating the
>> right exception, which is #GP rather than #UD), it distinguishes better
>> between page faults and too long instructions, as shown by this test case:
>>
>>     #include <sys/mman.h>
>>     #include <string.h>
>>     #include <stdio.h>
>>
>>     int main()
>>     {
>>             char *x = mmap(NULL, 8192, PROT_READ|PROT_WRITE|PROT_EXEC,
>>                            MAP_PRIVATE|MAP_ANON, -1, 0);
>>             memset(x, 0x66, 4096);
>>             x[4096] = 0x90;
>>             x[4097] = 0xc3;
>>             char *i = x + 4096 - 15;
>>             mprotect(x + 4096, 4096, PROT_READ|PROT_WRITE);
>>             ((void(*)(void)) i) ();
>>     }
>>
>> ... which produces a #GP without the mprotect, and a #PF with it.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>  target/i386/translate.c | 29 ++++++++++++++++++++++-------
>>  1 file changed, 22 insertions(+), 7 deletions(-)
> 
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> 
>> +    if (sigsetjmp(s->jmpbuf, 0) != 0) {
> 
> Any particular reason to use sigsetjmp(x, 0) instead of setjmp(x)?
> Certainly there are no signal frames that the longjmp will pass...

sigsetjmp is used to _not_ save the signal mask.  On OS X setjmp saves
the signal mask by default, which is slower.

Paolo