CVE-2024-26906

In the Linux kernel, the following vulnerability has been resolved:

x86/mm: Disallow vsyscall page read for copy_from_kernel_nofault()

When trying to use copy_from_kernel_nofault() to read vsyscall page through a bpf program, the following oops was reported:

BUG: unable to handle page fault for address: ffffffffff600000 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 3231067 P4D 3231067 PUD 3233067 PMD 3235067 PTE 0 Oops: 0000 [#1] PREEMPT SMP PTI CPU: 1 PID: 20390 Comm: test_progs …… 6.7.0+ #58 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) …… RIP: 0010:copy_from_kernel_nofault+0x6f/0x110 …… Call Trace: ? copy_from_kernel_nofault+0x6f/0x110 bpf_probe_read_kernel+0x1d/0x50 bpf_prog_2061065e56845f08_do_probe_read+0x51/0x8d trace_call_bpf+0xc5/0x1c0 perf_call_bpf_enter.isra.0+0x69/0xb0 perf_syscall_enter+0x13e/0x200 syscall_trace_enter+0x188/0x1c0 do_syscall_64+0xb5/0xe0 entry_SYSCALL_64_after_hwframe+0x6e/0x76 …… —[ end trace 0000000000000000 ]—

The oops is triggered when:

1. A bpf program uses bpf_probe_read_kernel() to read from the vsyscall page and invokes copy_from_kernel_nofault() which in turn calls __get_user_asm().

2. Because the vsyscall page address is not readable from kernel space, a page fault exception is triggered accordingly.

3. handle_page_fault() considers the vsyscall page address as a user space address instead of a kernel space address. This results in the fix-up setup by bpf not being applied and a page_fault_oops() is invoked due to SMAP.

Considering handle_page_fault() has already considered the vsyscall page address as a userspace address, fix the problem by disallowing vsyscall page read for copy_from_kernel_nofault().

Affected Software

References

• https://git.kernel.org/stable/c/29bd6f86904682adafe9affbc7f79b14defcaff8
• https://git.kernel.org/stable/c/32019c659ecfe1d92e3bf9fcdfbb11a7c70acd58
• https://git.kernel.org/stable/c/57f78c46f08198e1be08ffe99c4c1ccc12855bf5
• https://git.kernel.org/stable/c/6e4694e65b6db4c3de125115dd4f55848cc48381
• https://git.kernel.org/stable/c/e8a67fe34b76a49320b33032228a794f40b0316b
• https://git.kernel.org/stable/c/f175de546a3eb77614d94d4c02550181c0a8493e