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: 8a9949bc71a7 ("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>

diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
index a8a4f4c460a6cb5cf0b20329c356f0cde0ee1ca2..c5fee2680abc87989b2b18f7752eb2b5e949c04c 100644 (file)
--- a/arch/x86/xen/xen-asm_64.S
+++ b/arch/x86/xen/xen-asm_64.S
@@ -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)