From: Andy Lutomirski <luto@kernel.org>
Date: Sun, 24 Nov 2019 08:50:03 -0800
Subject: x86/entry/32: Fix FIXUP_ESPFIX_STACK with user CR3
Git-commit: 4a13b0e3e10996b9aa0b45a764ecfe49f6fcd360
Patch-mainline: v5.5-rc1
References: bnc#1151927 5.3.14
UNWIND_ESPFIX_STACK needs to read the GDT, and the GDT mapping that
can be accessed via %fs is not mapped in the user pagetables. Use
SGDT to find the cpu_entry_area mapping and read the espfix offset
from that instead.
Reported-and-tested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
arch/x86/entry/entry_32.S | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -415,7 +415,8 @@
.macro CHECK_AND_APPLY_ESPFIX
#ifdef CONFIG_X86_ESPFIX32
-#define GDT_ESPFIX_SS PER_CPU_VAR(gdt_page) + (GDT_ENTRY_ESPFIX_SS * 8)
+#define GDT_ESPFIX_OFFSET (GDT_ENTRY_ESPFIX_SS * 8)
+#define GDT_ESPFIX_SS PER_CPU_VAR(gdt_page) + GDT_ESPFIX_OFFSET
ALTERNATIVE "jmp .Lend_\@", "", X86_BUG_ESPFIX
@@ -1147,12 +1148,26 @@ ENDPROC(entry_INT80_32)
* We can't call C functions using the ESPFIX stack. This code reads
* the high word of the segment base from the GDT and swiches to the
* normal stack and adjusts ESP with the matching offset.
+ *
+ * We might be on user CR3 here, so percpu data is not mapped and we can't
+ * access the GDT through the percpu segment. Instead, use SGDT to find
+ * the cpu_entry_area alias of the GDT.
*/
#ifdef CONFIG_X86_ESPFIX32
/* fixup the stack */
- mov GDT_ESPFIX_SS + 4, %al /* bits 16..23 */
- mov GDT_ESPFIX_SS + 7, %ah /* bits 24..31 */
+ pushl %ecx
+ subl $2*4, %esp
+ sgdt (%esp)
+ movl 2(%esp), %ecx /* GDT address */
+ /*
+ * Careful: ECX is a linear pointer, so we need to force base
+ * zero. %cs is the only known-linear segment we have right now.
+ */
+ mov %cs:GDT_ESPFIX_OFFSET + 4(%ecx), %al /* bits 16..23 */
+ mov %cs:GDT_ESPFIX_OFFSET + 7(%ecx), %ah /* bits 24..31 */
shl $16, %eax
+ addl $2*4, %esp
+ popl %ecx
addl %esp, %eax /* the adjusted stack pointer */
pushl $__KERNEL_DS
pushl %eax