Commit fa2016a8 authored Aug 14, 2017 by Andy Lutomirski Committed by Ingo Molnar Aug 15, 2017

x86/xen/64: Fix the reported SS and CS in SYSCALL

When I cleaned up the Xen SYSCALL entries, I inadvertently changed
the reported segment registers.  Before my patch, regs->ss was
__USER(32)_DS and regs->cs was __USER(32)_CS.  After the patch, they
are FLAT_USER_CS/DS(32).

This had a couple unfortunate effects.  It confused the
opportunistic fast return logic.  It also significantly increased
the risk of triggering a nasty glibc bug:

  https://sourceware.org/bugzilla/show_bug.cgi?id=21269



Update the Xen entry code to change it back.

Reported-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: xen-devel@lists.xenproject.org
Fixes: 8a9949bc ("x86/xen/64: Rearrange the SYSCALL entries")
Link: http://lkml.kernel.org/r/daba8351ea2764bb30272296ab9ce08a81bd8264.1502775273.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent bf4d1a83

arch/x86/xen/xen-asm_64.S

+18 −0

Original line number	Diff line number	Diff line
		@@ -88,6 +88,15 @@ RELOC(xen_sysret64, 1b+1)
		ENTRY(xen_syscall_target)
		popq %rcx
		popq %r11

		/*
		* Neither Xen nor the kernel really knows what the old SS and
		* CS were. The kernel expects __USER_DS and __USER_CS, so
		* report those values even though Xen will guess its own values.
		*/
		movq $__USER_DS, 4*8(%rsp)
		movq $__USER_CS, 1*8(%rsp)

		jmp entry_SYSCALL_64_after_hwframe
		ENDPROC(xen_syscall_target)

		@@ -97,6 +106,15 @@ ENDPROC(xen_syscall_target)
		ENTRY(xen_syscall32_target)
		popq %rcx
		popq %r11

		/*
		* Neither Xen nor the kernel really knows what the old SS and
		* CS were. The kernel expects __USER32_DS and __USER32_CS, so
		* report those values even though Xen will guess its own values.
		*/
		movq $__USER32_DS, 4*8(%rsp)
		movq $__USER32_CS, 1*8(%rsp)

		jmp entry_SYSCALL_compat_after_hwframe
		ENDPROC(xen_syscall32_target)

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