From: Pawan Gupta Date: Thu, 26 Sep 2024 16:10:31 +0000 (-0700) Subject: x86/bugs: Use code segment selector for VERW operand X-Git-Url: http://git.hungrycats.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=bfd1d223d80cb29a210caa1bd5e21f0816d58f02;p=linux x86/bugs: Use code segment selector for VERW operand commit e4d2102018542e3ae5e297bc6e229303abff8a0f upstream. 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 Reviewed-by: Andrew Cooper Suggested-by: Brian Gerst Signed-off-by: Pawan Gupta Signed-off-by: Dave Hansen Signed-off-by: Ingo Molnar Signed-off-by: Greg Kroah-Hartman --- diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index 87e1ff064025..7978d5fe1ce6 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -199,7 +199,16 @@ */ .macro CLEAR_CPU_BUFFERS ALTERNATIVE "jmp .Lskip_verw_\@", "", X86_FEATURE_CLEAR_CPU_BUF - verw _ASM_RIP(mds_verw_sel) +#ifdef CONFIG_X86_64 + verw mds_verw_sel(%rip) +#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). + */ + verw %cs:mds_verw_sel +#endif .Lskip_verw_\@: .endm