On Fri, Nov 01, 2024 at 06:26:09PM +0800, Xiongfeng Wang wrote:
> From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
>
> 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 <rtgill82@xxxxxxxxx>
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 5.10+
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218707
> Closes: https://lore.kernel.org/all/8c77ccfd-d561-45a1-8ed5-6b75212c7a58@xxxxxxxxxxxxx/
> Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Suggested-by: Brian Gerst <brgerst@xxxxxxxxx>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> [xiongfeng: fix conflicts caused by the runtime patch jmp]
> Signed-off-by: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
> ---
> 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 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
I sent these backports sometime back, seems they were not picked:
5.10: https://lore.kernel.org/stable/f9c84ff992511890556cd52c19c2875b440b98c6.1729538774.git.pawan.kumar.gupta@xxxxxxxxxxxxxxx/
5.15: https://lore.kernel.org/stable/d2fca828795e4980e0708a179bd60b2a89bc8089.1729538132.git.pawan.kumar.gupta@xxxxxxxxxxxxxxx/
6.1: https://lore.kernel.org/stable/7aad4bddc4cf131ee88657da20960c4a714aa756.1729536596.git.pawan.kumar.gupta@xxxxxxxxxxxxxxx/
