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authorDavid Vrabel <david.vrabel@citrix.com>2012-10-19 20:29:07 +0400
committerKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>2012-10-19 23:17:59 +0400
commita349e23d1cf746f8bdc603dcc61fae9ee4a695f6 (patch)
tree49df313d4d1e37189e1ae6f052d11381e1443f80 /arch
parentee7b5958e2494619ee3ff52de68580feed6906a2 (diff)
downloadlinux-a349e23d1cf746f8bdc603dcc61fae9ee4a695f6.tar.xz
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.vrabel@citrix.com> Acked-by: Jan Beulich <JBeulich@suse.com> Acked-by: Ian Campbell <ian.campbell@citrix.com> Cc: stable@vger.kernel.org Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Diffstat (limited to 'arch')
-rw-r--r--arch/x86/kernel/entry_32.S8
-rw-r--r--arch/x86/kernel/entry_64.S2
2 files changed, 6 insertions, 4 deletions
diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 623f28837476..8f8e8eea9085 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 69babd8c834f..dcdd0ea33a32 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