Messages in this thread | | | Date | Sat, 4 Nov 2000 02:07:09 +0100 | From | Andrea Arcangeli <> | Subject | Re: SETFPXREGS fix |
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On Sat, Nov 04, 2000 at 10:50:00AM +1100, Gareth Hughes wrote: > if ( HAVE_FXSR ) { > if ( __copy_from_user( &tsk->thread.i387.fxsave, (void *)buf, > sizeof(struct user_fxsr_struct) ) ) > return -EFAULT; > /* bit 6 and 31-16 must be zero for security reasons */ > tsk->thread.i387.fxsave.mxcsr &= 0x0000ffbf; > return 0; > }
The above doesn't fix the security problem. Put the last byte of the userspace structure on an unmapped page and it will return -EFAULT lefting the invalid mxcsr value that will corrupt the FPU again.
The right version of the above is just in linux mailbox.
The reason I did it more complex at first is because I wanted to go safe, I wasn't sure if somebody could SIGCONT the traced task while we was copying the data so introducing a race where it was still possible to exploit the bug; but as Linus pointed out to me the loop in do_signal prevents that, so we can do only one large copy and then fixup (fixing up also in the -EFAULT case of course).
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