Re: [PATCH 3/6] x86/entry: Use PUSH_AND_CLEAR_REGS for compat
From: Josh Poimboeuf
Date: Thu May 19 2022 - 13:35:46 EST
On Thu, May 19, 2022 at 10:11:31AM -0700, Josh Poimboeuf wrote:
> On Thu, May 19, 2022 at 07:00:09PM +0200, Peter Zijlstra wrote:
> > On Thu, May 19, 2022 at 09:24:11AM -0700, Guenter Roeck wrote:
> > > On Fri, May 06, 2022 at 02:14:34PM +0200, Peter Zijlstra wrote:
> > > > Since the upper regs don't exist for ia32 code, preserving them
> > > > doesn't hurt and it simplifies the code.
> > > >
> > > > This doesn't add any attack surface that would not already be
> > > > available through INT80.
> > > >
> > > > Notably:
> > > >
> > > > - 32bit SYSENTER: didn't clear si, dx, cx.
> > > >
> > > > - 32bit SYSCALL, INT80: *do* clear si since the C functions don't
> > > > take a second argument.
> > > >
> > > > - 64bit: didn't clear si since the C functions take a second
> > > > argument; except the error_entry path might have only one argument,
> > > > so clearing si was missing here.
> > > >
> > > > 32b SYSENTER should be clearing all those 3 registers, nothing uses them
> > > > and selftests pass.
> > > >
> > > > Unconditionally clear rsi since it simplifies code.
> > > >
> > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > > > Reviewed-by: Borislav Petkov <bp@xxxxxxx>
> > >
> > > linux-next (next-20220519) crashes due to this patch when booting
> > > q35:EPYC-Rome in qemu.
> >
> > Could you try backing out each of the hunks one at a time? They're all
> > more or less independent.
> >
> > My bet with this being a #PF on an AMD machine, it's either the SI clear
> > or the SYSCALL change.
>
> I think this should fix it:
Actually that one had a horrendous bug :-)
Guenter, can you try this one?
From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Subject: [PATCH] x86/entry: Fix register corruption in compat syscall
A panic was reported in the init process on AMD:
Run /sbin/init as init process
init[1]: segfault at f7fd5ca0 ip 00000000f7f5bbc7 sp 00000000ffa06aa0 error 7 in libc.so[f7f51000+4e000]
Code: 8a 44 24 10 88 41 ff 8b 44 24 10 83 c4 2c 5b 5e 5f 5d c3 53 83 ec 08 8b 5c 24 10 81 fb 00 f0 ff ff 76 0c e8 ba dc ff ff f7 db <89> 18 83 cb ff 83 c4 08 89 d8 5b c3 e8 81 60 ff ff 05 28 84 07 00
Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
CPU: 1 PID: 1 Comm: init Tainted: G W 5.18.0-rc7-next-20220519 #1
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0x57/0x7d
panic+0x10f/0x28d
do_exit.cold+0x18/0x48
do_group_exit+0x2e/0xb0
get_signal+0xb6d/0xb80
arch_do_signal_or_restart+0x31/0x760
? show_opcodes.cold+0x1c/0x21
? force_sig_fault+0x49/0x70
exit_to_user_mode_prepare+0x131/0x1a0
irqentry_exit_to_user_mode+0x5/0x30
asm_exc_page_fault+0x27/0x30
RIP: 0023:0xf7f5bbc7
Code: 8a 44 24 10 88 41 ff 8b 44 24 10 83 c4 2c 5b 5e 5f 5d c3 53 83 ec 08 8b 5c 24 10 81 fb 00 f0 ff ff 76 0c e8 ba dc ff ff f7 db <89> 18 83 cb ff 83 c4 08 89 d8 5b c3 e8 81 60 ff ff 05 28 84 07 00
RSP: 002b:00000000ffa06aa0 EFLAGS: 00000217
RAX: 00000000f7fd5ca0 RBX: 000000000000000c RCX: 0000000000001000
RDX: 0000000000000001 RSI: 00000000f7fd5b60 RDI: 00000000f7fd5b60
RBP: 00000000f7fd1c1c R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
</TASK>
The task's CX register got corrupted by commit 8c42819b61b8 ("x86/entry:
Use PUSH_AND_CLEAR_REGS for compat"), which overlooked the fact that
compat SYSCALL had the user's CX value stored in BP.
Before that commit, CX was saved from its stashed value in BP:
pushq %rbp /* pt_regs->cx (stashed in bp) */
But then it got changed to:
pushq %rcx /* pt_regs->cx */
resulting in the wrong value getting saved and later restored back to
the user. Fix it by pushing the correct value again (BP) for regs->cx.
Fixes: 8c42819b61b8 ("x86/entry: Use PUSH_AND_CLEAR_REGS for compat")
Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
arch/x86/entry/calling.h | 8 ++++----
arch/x86/entry/entry_64_compat.S | 2 +-
2 files changed, 5 insertions(+), 5 deletions(-)
diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index a97cc78ecb92..29b36e9e4e74 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -63,7 +63,7 @@ For 32-bit we have the following conventions - kernel is built with
* for assembly code:
*/
-.macro PUSH_REGS rdx=%rdx rax=%rax save_ret=0
+.macro PUSH_REGS rdx=%rdx rcx=%rcx rax=%rax save_ret=0
.if \save_ret
pushq %rsi /* pt_regs->si */
movq 8(%rsp), %rsi /* temporarily store the return address in %rsi */
@@ -73,7 +73,7 @@ For 32-bit we have the following conventions - kernel is built with
pushq %rsi /* pt_regs->si */
.endif
pushq \rdx /* pt_regs->dx */
- pushq %rcx /* pt_regs->cx */
+ pushq \rcx /* pt_regs->cx */
pushq \rax /* pt_regs->ax */
pushq %r8 /* pt_regs->r8 */
pushq %r9 /* pt_regs->r9 */
@@ -115,8 +115,8 @@ For 32-bit we have the following conventions - kernel is built with
.endm
-.macro PUSH_AND_CLEAR_REGS rdx=%rdx rax=%rax save_ret=0
- PUSH_REGS rdx=\rdx, rax=\rax, save_ret=\save_ret
+.macro PUSH_AND_CLEAR_REGS rdx=%rdx rcx=%rcx rax=%rax save_ret=0
+ PUSH_REGS rdx=\rdx, rcx=\rcx, rax=\rax, save_ret=\save_ret
CLEAR_REGS
.endm
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index ed2be3615b50..f76e674d22c4 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -200,7 +200,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_safe_stack, SYM_L_GLOBAL)
SYM_INNER_LABEL(entry_SYSCALL_compat_after_hwframe, SYM_L_GLOBAL)
movl %eax, %eax /* discard orig_ax high bits */
pushq %rax /* pt_regs->orig_ax */
- PUSH_AND_CLEAR_REGS rax=$-ENOSYS
+ PUSH_AND_CLEAR_REGS rcx=%rbp rax=$-ENOSYS
UNWIND_HINT_REGS
movq %rsp, %rdi
--
2.34.3