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().

Notes