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:

<TASK>

? 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

</TASK>

......

---[ 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().