On Fri, Feb 2, 2024 at 11:03 AM Sohil Mehta <sohil.mehta@xxxxxxxxx> wrote: > > On 2/2/2024 2:39 AM, Hou Tao wrote: > > From: Hou Tao <houtao1@xxxxxxxxxx> > > > > 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(). > > > > Originally-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Thomas, could you please Ack the patch if you're still ok with it, so we can take through the bpf tree to Linus soon ? Not only syzbot, but real users are hitting this bug. Thanks! > > Reported-by: syzbot+72aa0161922eba61b50e@xxxxxxxxxxxxxxxxxxxxxxxxx > > Closes: https://lore.kernel.org/bpf/CAG48ez06TZft=ATH1qh2c5mpS5BT8UakwNkzi6nvK5_djC-4Nw@xxxxxxxxxxxxxx > > Reported-by: xingwei lee <xrivendell7@xxxxxxxxx> > > Closes: https://lore.kernel.org/bpf/CABOYnLynjBoFZOf3Z4BhaZkc5hx_kHfsjiW+UWLoB=w33LvScw@xxxxxxxxxxxxxx > > Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx> > > --- > > arch/x86/mm/maccess.c | 10 ++++++++++ > > 1 file changed, 10 insertions(+) > > > > Reviewed-by: Sohil Mehta <sohil.mehta@xxxxxxxxx> >
