commit: a349e23d1cf746f8bdc603dcc61fae9ee4a695f6
From: David Vrabel <david.vra...@citrix.com>
Date: Fri, 19 Oct 2012 17:29:07 +0100
Subject: xen/x86: don't corrupt %eip when returning from a signal handler
In 32 bit guests, if a userspace process has %eax == -ERESTARTSYS
(-512) or -ERESTARTNOINTR (-513) when it is interrupted by an event
/and/ the process has a pending signal then %eip (and %eax) are
corrupted when returning to the main process after handling the
signal. The application may then crash with SIGSEGV or a SIGILL or it
may have subtly incorrect behaviour (depending on what instruction it
returned to).
The occurs because handle_signal() is incorrectly thinking that there
is a system call that needs to restarted so it adjusts %eip and %eax
to re-execute the system call instruction (even though user space had
not done a system call).
If %eax == -514 (-ERESTARTNOHAND (-514) or -ERESTART_RESTARTBLOCK
(-516) then handle_signal() only corrupted %eax (by setting it to
-EINTR). This may cause the application to crash or have incorrect
behaviour.
handle_signal() assumes that regs->orig_ax >= 0 means a system call so
any kernel entry point that is not for a system call must push a
negative value for orig_ax. For example, for physical interrupts on
bare metal the inverse of the vector is pushed and page_fault() sets
regs->orig_ax to -1, overwriting the hardware provided error code.
xen_hypervisor_callback() was incorrectly pushing 0 for orig_ax
instead of -1.
Classic Xen kernels pushed %eax which works as %eax cannot be both
non-negative and -RESTARTSYS (etc.), but using -1 is consistent with
other non-system call entry points and avoids some of the tests in
handle_signal().
There were similar bugs in xen_failsafe_callback() of both 32 and
64-bit guests. If the fault was corrected and the normal return path
was used then 0 was incorrectly pushed as the value for orig_ax.
Signed-off-by: David Vrabel <david.vra...@citrix.com>
Acked-by: Jan Beulich <jbeul...@suse.com>
Acked-by: Ian Campbell <ian.campb...@citrix.com>
Cc: stable@vger.kernel.org
Signed-off-by: Konrad Rzeszutek Wilk <konrad.w...@oracle.com>
---
arch/x86/kernel/entry_32.S | 8 +++++---
arch/x86/kernel/entry_64.S | 2 +-
2 files changed, 6 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 623f288..8f8e8ee 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -1016,7 +1016,7 @@ ENTRY(xen_sysenter_target)
ENTRY(xen_hypervisor_callback)
CFI_STARTPROC
- pushl_cfi $0
+ pushl_cfi $-1 /* orig_ax = -1 => not a system call */
SAVE_ALL
TRACE_IRQS_OFF
@@ -1058,14 +1058,16 @@ ENTRY(xen_failsafe_callback)
2: mov 8(%esp),%es
3: mov 12(%esp),%fs
4: mov 16(%esp),%gs
+ /* EAX == 0 => Category 1 (Bad segment)
+ EAX != 0 => Category 2 (Bad IRET) */
testl %eax,%eax
popl_cfi %eax
lea 16(%esp),%esp
CFI_ADJUST_CFA_OFFSET -16
jz 5f
addl $16,%esp
- jmp iret_exc # EAX != 0 => Category 2 (Bad IRET)
-5: pushl_cfi $0 # EAX == 0 => Category 1 (Bad segment)
+ jmp iret_exc
+5: pushl_cfi $-1 /* orig_ax = -1 => not a system call */
SAVE_ALL
jmp ret_from_exception
CFI_ENDPROC
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 69babd8..dcdd0ea 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1363,7 +1363,7 @@ ENTRY(xen_failsafe_callback)
CFI_RESTORE r11
addq $0x30,%rsp
CFI_ADJUST_CFA_OFFSET -0x30
- pushq_cfi $0
+ pushq_cfi $-1 /* orig_ax = -1 => not a system call */
SAVE_ALL
jmp error_exit
CFI_ENDPROC
--
1.7.3.4
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