The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: e4d2102018542e3ae5e297bc6e229303abff8a0f Gitweb: https://git.kernel.org/tip/e4d2102018542e3ae5e297bc6e229303abff8a0f Author: Pawan Gupta pawan.kumar.gupta@linux.intel.com AuthorDate: Thu, 26 Sep 2024 09:10:31 -07:00 Committer: Ingo Molnar mingo@kernel.org CommitterDate: Wed, 09 Oct 2024 09:42:30 +02:00

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.

[ mingo: Fixed the SOB chain. ]

Fixes: a0e2dab44d22 ("x86/entry_32: Add VERW just before userspace transition") Reported-by: Robert Gill rtgill82@gmail.com Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com Cc: stable@vger.kernel.org # 5.10+ 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 Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Signed-off-by: Ingo Molnar mingo@kernel.org --- 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 ff5f1ec..96b410b 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 .endm

#ifdef CONFIG_X86_64