linux-kernel - Re: 32bit binaries on x86_64/Xen segfaults in syscall-vdso

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 04 Sep 2009 09:07:39 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Bastian Blank <waldi@...ian.org>, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com, 544145@...s.debian.org,
	Keir Fraser <keir.fraser@...citrix.com>
Subject: Re: 32bit binaries on x86_64/Xen segfaults in syscall-vdso

On 09/03/09 15:36, Bastian Blank wrote:
> This function looks weird. It tries to restores the user code segment.
> But the documentation from AMD explicitely stat that the CS and SS are
> restored from the STAR register.

And STAR is always set with:

    wrmsrl(MSR_STAR,  ((u64)__USER32_CS)<<48  | ((u64)__KERNEL_CS)<<32);

so when using sysret to return to 32-bit, it:

    The CS selector value is set to MSR IA32_STAR[63:48]. The SS is set
    to IA32_STAR[63:48] + 8.

so CS is __USER32_CS and SS is __USER32_DS.

The code for xen_sysret32 is:

ENTRY(xen_sysret32)
	/*
	 * We're already on the usermode stack at this point, but
	 * still with the kernel gs, so we can easily switch back
	 */
	movq %rsp, PER_CPU_VAR(old_rsp)
	movq PER_CPU_VAR(kernel_stack), %rsp

	pushq $__USER32_DS
	pushq PER_CPU_VAR(old_rsp)
	pushq %r11
	pushq $__USER32_CS
	pushq %rcx

	pushq $VGCF_in_syscall
1:	jmp hypercall_iret

The iret frame is:

 	ss
 	rsp
 	rflags
 	cs
 	rip		<-- rsp

so this constructs a frame of:

	__USER32_DS
	user_esp
	user_eflags
	__USER32_CS
	user_eip	<-- kernel rsp

and then it does the iret hypercall.

But for some reason that's triggering a failsafe callback, which invokes
a GP.

    J
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/