On 26/09/2024 5:10 pm, Pawan Gupta wrote:
On Thu, Sep 26, 2024 at 04:52:53PM +0200, Uros Bizjak wrote:
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index e18a6aaf414c..4228a1fd2c2e 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -318,14 +318,21 @@ /*
- Macro to execute VERW instruction that mitigate transient data sampling
- attacks such as MDS. On affected systems a microcode update overloaded VERW
- instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF. Using %cs
- to reference VERW operand avoids a #GP fault for an arbitrary user %ds in
- 32-bit mode.
*/ .macro CLEAR_CPU_BUFFERS
- instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF.
- Note: Only the memory operand variant of VERW clears the CPU buffers.
- ALTERNATIVE "", __stringify(verw %cs:_ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
+#ifdef CONFIG_X86_64
- ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
You should drop _ASM_RIP here and direclty use (%rip). This way, you also won't need __stringify:
ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF
+#else
- /*
* In 32bit mode, the memory operand must be a %cs reference. The data* segments may not be usable (vm86 mode), and the stack segment may not* be flat (ESPFIX32).*/- ALTERNATIVE "", __stringify(verw %cs:mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF
Also here, no need for __stringify:
ALTERNATIVE "", "verw %cs:mds_verw_sel", X86_FEATURE_CLEAR_CPU_BUF
This is in fact what Andrew proposed in his review.
Thanks for pointing out, I completely missed that part. Below is how it looks like with stringify gone:
--- >8 --- Subject: [PATCH] x86/bugs: Use code segment selector for VERW operand
Robert Gill reported below #GP in 32-bit mode when dosemu software was executing vm86() system call:
general protection fault: 0000 [#1] PREEMPT SMP CPU: 4 PID: 4610 Comm: dosemu.bin Not tainted 6.6.21-gentoo-x86 #1 Hardware name: Dell Inc. PowerEdge 1950/0H723K, BIOS 2.7.0 10/30/2010 EIP: restore_all_switch_stack+0xbe/0xcf EAX: 00000000 EBX: 00000000 ECX: 00000000 EDX: 00000000 ESI: 00000000 EDI: 00000000 EBP: 00000000 ESP: ff8affdc DS: 0000 ES: 0000 FS: 0000 GS: 0033 SS: 0068 EFLAGS: 00010046 CR0: 80050033 CR2: 00c2101c CR3: 04b6d000 CR4: 000406d0 Call Trace: show_regs+0x70/0x78 die_addr+0x29/0x70 exc_general_protection+0x13c/0x348 exc_bounds+0x98/0x98 handle_exception+0x14d/0x14d exc_bounds+0x98/0x98 restore_all_switch_stack+0xbe/0xcf exc_bounds+0x98/0x98 restore_all_switch_stack+0xbe/0xcf
This only happens in 32-bit mode when VERW based mitigations like MDS/RFDS are enabled. This is because segment registers with an arbitrary user value can result in #GP when executing VERW. Intel SDM vol. 2C documents the following behavior for VERW instruction:
#GP(0) - If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
CLEAR_CPU_BUFFERS macro executes VERW instruction before returning to user space. Use %cs selector to reference VERW operand. This ensures VERW will not #GP for an arbitrary user %ds.
Fixes: a0e2dab44d22 ("x86/entry_32: Add VERW just before userspace transition") Cc: stable@vger.kernel.org # 5.10+ Reported-by: Robert Gill rtgill82@gmail.com Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218707 Closes: https://lore.kernel.org/all/8c77ccfd-d561-45a1-8ed5-6b75212c7a58@leemhuis.in... Suggested-by: Dave Hansen dave.hansen@linux.intel.com Suggested-by: Brian Gerst brgerst@gmail.com Signed-off-by: Pawan Gupta pawan.kumar.gupta@linux.intel.com
arch/x86/include/asm/nospec-branch.h | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index ff5f1ecc7d1e..96b410b1d4e8 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -323,7 +323,16 @@
- Note: Only the memory operand variant of VERW clears the CPU buffers.
*/ .macro CLEAR_CPU_BUFFERS
- ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
+#ifdef CONFIG_X86_64
- ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF
+#else
- /*
* In 32bit mode, the memory operand must be a %cs reference. The data* segments may not be usable (vm86 mode), and the stack segment may not* be flat (ESPFIX32).*/- ALTERNATIVE "", "verw %cs:mds_verw_sel", X86_FEATURE_CLEAR_CPU_BUF
+#endif
You should also delete _ASM_RIP() as you're removing the only user of it.
But yes, with that, Reviewed-by: Andrew Cooper andrew.cooper3@citrix.com FWIW.