When running with return thunks enabled under 32-bit EFI, the system
crashes with:
[ 0.137688] kernel tried to execute NX-protected page - exploit attempt? (uid: 0)
[ 0.138136] BUG: unable to handle page fault for address: 000000005bc02900
[ 0.138136] #PF: supervisor instruction fetch in kernel mode
[ 0.138136] #PF: error_code(0x0011) - permissions violation
[ 0.138136] PGD 18f7063 P4D 18f7063 PUD 18ff063 PMD 190e063 PTE 800000005bc02063
[ 0.138136] Oops: 0011 [#1] PREEMPT SMP PTI
[ 0.138136] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.19.0-rc6+ #166
[ 0.138136] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
[ 0.138136] RIP: 0010:0x5bc02900
[ 0.138136] Code: Unable to access opcode bytes at RIP 0x5bc028d6.
[ 0.138136] RSP: 0018:ffffffffb3203e10 EFLAGS: 00010046
[ 0.138136] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000048
[ 0.138136] RDX: 000000000190dfac RSI: 0000000000001710 RDI: 000000007eae823b
[ 0.138136] RBP: ffffffffb3203e70 R08: 0000000001970000 R09: ffffffffb3203e28
[ 0.138136] R10: 747563657865206c R11: 6c6977203a696665 R12: 0000000000001710
[ 0.138136] R13: 0000000000000030 R14: 0000000001970000 R15: 0000000000000001
[ 0.138136] FS: 0000000000000000(0000) GS:ffff8e013ca00000(0000) knlGS:0000000000000000
[ 0.138136] CS: 0010 DS: 0018 ES: 0018 CR0: 0000000080050033
[ 0.138136] CR2: 000000005bc02900 CR3: 0000000001930000 CR4: 00000000000006f0
[ 0.138136] Call Trace:
[ 0.138136] <TASK>
[ 0.138136] ? efi_set_virtual_address_map+0x9c/0x175
[ 0.138136] efi_enter_virtual_mode+0x4a6/0x53e
[ 0.138136] start_kernel+0x67c/0x71e
[ 0.138136] x86_64_start_reservations+0x24/0x2a
[ 0.138136] x86_64_start_kernel+0xe9/0xf4
[ 0.138136] secondary_startup_64_no_verify+0xe5/0xeb
[ 0.138136] </TASK>
That's because it cannot jump to the return thunk from the 32-bit code.
Using a naked RET and marking it as safe allows the system to proceed
booting.
Fixes: aa3d480315ba ("x86: Use return-thunk in asm code")
Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
Does this leave one potential attack vector open? Perhaps, since this is
running under a different mapping (AFAIU), the risk is reduced? Or rather, the
attacker could attack using the firmware RETs anyway?
Alternatively, we could use IBPB when available when using the wrapper.
Thoughts?
---
arch/x86/platform/efi/efi_thunk_64.S | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/x86/platform/efi/efi_thunk_64.S b/arch/x86/platform/efi/efi_thunk_64.S
index 9ffe2bad27d5..4e5257a4811b 100644
--- a/arch/x86/platform/efi/efi_thunk_64.S
+++ b/arch/x86/platform/efi/efi_thunk_64.S
@@ -23,6 +23,7 @@
#include <linux/objtool.h>
#include <asm/page_types.h>
#include <asm/segment.h>
+#include <asm/nospec-branch.h>
.text
.code64
@@ -75,7 +76,9 @@ STACK_FRAME_NON_STANDARD __efi64_thunk
1: movq 0x20(%rsp), %rsp
pop %rbx
pop %rbp
- RET
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
.code32
2: pushl $__KERNEL_CS
--
2.34.1
