CVE request: kernel: x86-64: seccomp: 32/64 syscall hole

[Eugene Teo <eugeneteo () kernel sg>]

On x86-64, a 32-bit process (TIF_IA32) can switch to 64-bit mode with
ljmp, and then use the "syscall" instruction to make a 64-bit system
call.  A 64-bit process make a 32-bit system call with int $0x80.

In both these cases under CONFIG_SECCOMP=y, secure_computing() will
use the wrong system call number table.  The fix is simple: test
TS_COMPAT instead of TIF_IA32.

Credit: Chris Evans.

References:
https://bugzilla.redhat.com/show_bug.cgi?id=487255
http://scary.beasts.org/security/CESA-2009-001.html
http://scary.beasts.org/security/CESA-2009-004.html
http://lkml.org/lkml/2009/2/27/451 summary
http://lkml.org/lkml/2009/2/27/452 syscall-audit
http://lkml.org/lkml/2009/2/27/453 seccomp
http://lkml.org/lkml/2009/2/28/23 seccomp follow-ups