Re: [musl] i386 __set_thread_area will crash if the syscall fails

[Rich Felker <dalias@libc.org>]

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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

[-- Attachment #2: __set_thread_area.c --]
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#define SYSCALL_NO_TLS 1
#include <stdint.h>
#include "syscall.h"

struct user_desc {
	uint32_t entry_number;
	uint32_t base_addr;
	uint32_t limit;
	uint32_t flags;
};

static int entry_number = -1;

int __set_thread_area_2(void *p)
{
	struct user_desc desc = {
		entry_number, (uintptr_t)p, 0xfffff, 0x51
	};
	int r = __syscall(SYS_set_thread_area, &desc);
	if (!r) {
		entry_number = desc.entry_number;
		__asm__ __volatile__ ("mov %0,%%gs" :
			: "r"(3+8*desc.entry_number));
		return 0;
	}
	desc.entry_number = 0;
	r = __syscall(SYS_modify_ldt, 1, &desc, 16);
	if (!r) {
		__asm__ __volatile__ ("mov %0,%%gs" :
			: "r"(7));
		return 1;
	}
	return r;
}