[PATCH bpf v2 2/3] x86/mm: Disallow vsyscall page read for copy_from_kernel_nofault()

[Hou Tao]

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