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authorAndy Lutomirski <luto@amacapital.net>2014-11-22 18:00:33 -0800
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2014-12-06 15:55:33 -0800
commitfd2375c314545a7ce29f05b5b3889b171ba5fd39 (patch)
treef3d820642e1813cdf8c6e56191fd4e8af7014906
parentc6328855c41c28b2a53c7c6821af60dd3b41ddba (diff)
x86_64, traps: Rework bad_iret
commit b645af2d5905c4e32399005b867987919cbfc3ae upstream. It's possible for iretq to userspace to fail. This can happen because of a bad CS, SS, or RIP. Historically, we've handled it by fixing up an exception from iretq to land at bad_iret, which pretends that the failed iret frame was really the hardware part of #GP(0) from userspace. To make this work, there's an extra fixup to fudge the gs base into a usable state. This is suboptimal because it loses the original exception. It's also buggy because there's no guarantee that we were on the kernel stack to begin with. For example, if the failing iret happened on return from an NMI, then we'll end up executing general_protection on the NMI stack. This is bad for several reasons, the most immediate of which is that general_protection, as a non-paranoid idtentry, will try to deliver signals and/or schedule from the wrong stack. This patch throws out bad_iret entirely. As a replacement, it augments the existing swapgs fudge into a full-blown iret fixup, mostly written in C. It's should be clearer and more correct. Signed-off-by: Andy Lutomirski <luto@amacapital.net> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--arch/x86/kernel/entry_64.S45
-rw-r--r--arch/x86/kernel/traps.c29
2 files changed, 48 insertions, 26 deletions
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 29e2ff3bd5a7..02553d6d183d 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1055,8 +1055,13 @@ ENTRY(native_iret)
.global native_irq_return_iret
native_irq_return_iret:
+ /*
+ * This may fault. Non-paranoid faults on return to userspace are
+ * handled by fixup_bad_iret. These include #SS, #GP, and #NP.
+ * Double-faults due to espfix64 are handled in do_double_fault.
+ * Other faults here are fatal.
+ */
iretq
- _ASM_EXTABLE(native_irq_return_iret, bad_iret)
#ifdef CONFIG_X86_ESPFIX64
native_irq_return_ldt:
@@ -1084,25 +1089,6 @@ native_irq_return_ldt:
jmp native_irq_return_iret
#endif
- .section .fixup,"ax"
-bad_iret:
- /*
- * The iret traps when the %cs or %ss being restored is bogus.
- * We've lost the original trap vector and error code.
- * #GPF is the most likely one to get for an invalid selector.
- * So pretend we completed the iret and took the #GPF in user mode.
- *
- * We are now running with the kernel GS after exception recovery.
- * But error_entry expects us to have user GS to match the user %cs,
- * so swap back.
- */
- pushq $0
-
- SWAPGS
- jmp general_protection
-
- .previous
-
/* edi: workmask, edx: work */
retint_careful:
CFI_RESTORE_STATE
@@ -1629,16 +1615,15 @@ error_sti:
/*
* There are two places in the kernel that can potentially fault with
- * usergs. Handle them here. The exception handlers after iret run with
- * kernel gs again, so don't set the user space flag. B stepping K8s
- * sometimes report an truncated RIP for IRET exceptions returning to
- * compat mode. Check for these here too.
+ * usergs. Handle them here. B stepping K8s sometimes report a
+ * truncated RIP for IRET exceptions returning to compat mode. Check
+ * for these here too.
*/
error_kernelspace:
incl %ebx
leaq native_irq_return_iret(%rip),%rcx
cmpq %rcx,RIP+8(%rsp)
- je error_swapgs
+ je error_bad_iret
movl %ecx,%eax /* zero extend */
cmpq %rax,RIP+8(%rsp)
je bstep_iret
@@ -1649,7 +1634,15 @@ error_kernelspace:
bstep_iret:
/* Fix truncated RIP */
movq %rcx,RIP+8(%rsp)
- jmp error_swapgs
+ /* fall through */
+
+error_bad_iret:
+ SWAPGS
+ mov %rsp,%rdi
+ call fixup_bad_iret
+ mov %rax,%rsp
+ decl %ebx /* Return to usergs */
+ jmp error_sti
CFI_ENDPROC
END(error_entry)
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 9391caeac92d..f9d976e0ae67 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -384,6 +384,35 @@ asmlinkage __kprobes struct pt_regs *sync_regs(struct pt_regs *eregs)
*regs = *eregs;
return regs;
}
+
+struct bad_iret_stack {
+ void *error_entry_ret;
+ struct pt_regs regs;
+};
+
+asmlinkage __visible
+struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
+{
+ /*
+ * This is called from entry_64.S early in handling a fault
+ * caused by a bad iret to user mode. To handle the fault
+ * correctly, we want move our stack frame to task_pt_regs
+ * and we want to pretend that the exception came from the
+ * iret target.
+ */
+ struct bad_iret_stack *new_stack =
+ container_of(task_pt_regs(current),
+ struct bad_iret_stack, regs);
+
+ /* Copy the IRET target to the new stack. */
+ memmove(&new_stack->regs.ip, (void *)s->regs.sp, 5*8);
+
+ /* Copy the remainder of the stack from the current stack. */
+ memmove(new_stack, s, offsetof(struct bad_iret_stack, regs.ip));
+
+ BUG_ON(!user_mode_vm(&new_stack->regs));
+ return new_stack;
+}
#endif
/*