CVE-2024-50072 Information

Description

In the Linux kernel the following vulnerability has been resolved:

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. ]

Reference

https://git.kernel.org/stable/c/481b477ab63c7245715a3e57ba79eb87c2dc0d02 https://git.kernel.org/stable/c/bc576fbaf82deded606e69a00efe9752136bf91d https://git.kernel.org/stable/c/e4d2102018542e3ae5e297bc6e229303abff8a0f