Commit-ID: fa2016a8e7d846b306e431646d250500e1da0c33
Gitweb: http://git.kernel.org/tip/fa2016a8e7d846b306e431646d250500e1da0c33
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Mon, 14 Aug 2017 22:36:19 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 15 Aug 2017 10:10:58 +0200
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@xxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Fixes: 8a9949bc71a7 ("x86/xen/64: Rearrange the SYSCALL entries")
Link: http://lkml.kernel.org/r/daba8351ea2764bb30272296ab9ce08a81bd8264.1502775273.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/xen/xen-asm_64.S | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
index a8a4f4c..c5fee26 100644
--- 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)
--
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