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Date: Sun, 30 Aug 2020 22:51:38 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Cc: Theodore Dubois <tblodt@...oud.com>
Subject: Re: i386 __set_thread_area will crash if the syscall fails
On Sun, Aug 30, 2020 at 09:41:45PM -0400, Rich Felker wrote:
> On Sun, Aug 30, 2020 at 09:07:10PM -0400, Rich Felker wrote:
> > On Sun, Aug 30, 2020 at 05:34:09PM -0700, Theodore Dubois wrote:
> > > Found a (small) bug in this file:
> > > https://git.musl-libc.org/cgit/musl/tree/src/thread/i386/__set_thread_area..s
> > >
> > > If the syscall fails, the branch on line 20 is taken and %eax will
> > > be a small negative number. Then "mov $123,%al" will make syscall
> > > 0xffffff7b instead of 0x7b, since overwriting %al only overwrites
> > > the low byte of %eax. So the modify_ldt fallback has apparently
> > > never worked.
> >
> > Thanks! Indeed, systems where the first syscall fails are outside the
> > actually-supported version range (before 2.6) so it's likely that this
> > was never actually tested. Nowadays SECCOMP_FILTER makes it easy to
> > test, so we should actually try to test some of these things. Have you
> > checked if it works adding xor %eax,%eax before the byte mov or
> > changing it to a 32-bit mov?
>
> OK, I just confirmed it works after this change. I'll push a fix soon
> along with a bunch of other pending commits. Thanks again for the
> report.
The attached should be the equivalent in C, but it's somewhat larger.
Somewhat indifferent on replacing it.
Rich
View attachment "__set_thread_area.c" of type "text/plain" (658 bytes)
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