On Thu, Sep 26, 2024 at 12:29:00AM +0100, Andrew Cooper wrote:
On 25/09/2024 11:25 pm, Pawan Gupta wrote:
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 | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index ff5f1ecc7d1e..e18a6aaf414c 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -318,12 +318,14 @@ /*
- 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.
- 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.
- 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
- ALTERNATIVE "", __stringify(verw %cs:_ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
.endm
People ought rightly to double-take at this using %cs and not %ss. There is a good reason, but it needs describing explicitly. May I suggest the following:
*...
- 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). *...
Thanks for the suggestion. I will include this.
.macro CLEAR_CPU_BUFFERS #ifdef __x86_64__ ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF #else ALTERNATIVE "", "verw %cs:mds_verw_sel", X86_FEATURE_CLEAR_CPU_BUF #endif .endm
This also lets you drop _ASM_RIP(). It's a cute idea, but is more confusion than it's worth, because there's no such thing in 32bit mode.
"%cs:_ASM_RIP(mds_verw_sel)" reads as if it does nothing, because it really doesn't in 64bit mode.
Right, will drop _ASM_RIP() in 32-bit mode and %cs in 64-bit mode.