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 ]--- It seems the occurrence of oops depends on SMAP feature of CPU. It happens as follow: a bpf program uses bpf_probe_read_kernel() to read from vsyscall page, bpf_probe_read_kernel() invokes copy_from_kernel_nofault() in turn and then invokes __get_user_asm(). Because the vsyscall page address is not readable for kernel space, a page fault exception is triggered accordingly, handle_page_fault() considers the vsyscall page address as a userspace address instead of a kernel space address, so the fix-up set-up by bpf isn't applied. Because the CPU has SMAP feature and the access happens in kernel mode, so page_fault_oops() is invoked and an oops happens. If these is no SMAP feature, the fix-up set-up by bpf will be applied and copy_from_kernel_nofault() will return -EFAULT instead. 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> 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 | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c index 6993f026adec9..d9272e1db5224 100644 --- a/arch/x86/mm/maccess.c +++ b/arch/x86/mm/maccess.c @@ -3,6 +3,8 @@ #include <linux/uaccess.h> #include <linux/kernel.h> +#include <asm/vsyscall.h> + #ifdef CONFIG_X86_64 bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size) { @@ -15,6 +17,13 @@ bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size) if (vaddr < TASK_SIZE_MAX + PAGE_SIZE) return false; + /* Also consider the vsyscall page as userspace address. Otherwise, + * reading the vsyscall page in copy_from_kernel_nofault() may + * trigger an oops due to an unhandled page fault. + */ + if (is_vsyscall_vaddr(vaddr)) + return false; + /* * Allow everything during early boot before 'x86_virt_bits' * is initialized. Needed for instruction decoding in early -- 2.29.2
