[bpf,v3,2/3] x86/mm: Disallow vsyscall page read for copy_from_kernel_nofault()

Commit Message

Hou Tao Feb. 2, 2024, 10:39 a.m. UTC

From: Hou Tao <[e-mail]>

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 <[e-mail]>
Reported-by: [e-mail]
Closes: https://lore.kernel.org/bpf/CAG48ez06TZft=[e-mail]
Reported-by: xingwei lee <[e-mail]>
Closes: https://lore.kernel.org/bpf/CABOYnLynjBoFZOf3Z4BhaZkc5hx_kHfsjiW+UWLoB=[e-mail]
Signed-off-by: Hou Tao <[e-mail]>
---
 arch/x86/mm/maccess.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

Comments

Sohil Mehta Feb. 2, 2024, 6:43 p.m. UTC | #1

On 2/2/2024 2:39 AM, Hou Tao wrote:
> From: Hou Tao <[e-mail]>
> 
> 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 <[e-mail]>
> Reported-by: [e-mail]
> Closes: https://lore.kernel.org/bpf/CAG48ez06TZft=[e-mail]
> Reported-by: xingwei lee <[e-mail]>
> Closes: https://lore.kernel.org/bpf/CABOYnLynjBoFZOf3Z4BhaZkc5hx_kHfsjiW+UWLoB=[e-mail]
> Signed-off-by: Hou Tao <[e-mail]>
> ---
>  arch/x86/mm/maccess.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 

Reviewed-by: Sohil Mehta <[e-mail]>

Alexei Starovoitov Feb. 14, 2024, 10:51 p.m. UTC | #2

On Fri, Feb 2, 2024 at 11:03 AM Sohil Mehta <[e-mail]> wrote:
>
> On 2/2/2024 2:39 AM, Hou Tao wrote:
> > From: Hou Tao <[e-mail]>
> >
> > 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 <[e-mail]>

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: [e-mail]
> > Closes: https://lore.kernel.org/bpf/CAG48ez06TZft=[e-mail]
> > Reported-by: xingwei lee <[e-mail]>
> > Closes: https://lore.kernel.org/bpf/CABOYnLynjBoFZOf3Z4BhaZkc5hx_kHfsjiW+UWLoB=[e-mail]
> > Signed-off-by: Hou Tao <[e-mail]>
> > ---
> >  arch/x86/mm/maccess.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
>
> Reviewed-by: Sohil Mehta <[e-mail]>
>

Thomas Gleixner Feb. 15, 2024, 11:13 a.m. UTC | #3

On Fri, Feb 02 2024 at 18:39, Hou Tao wrote:
>
> Originally-by: Thomas Gleixner <[e-mail]>
> Reported-by: [e-mail]
> Closes: https://lore.kernel.org/bpf/CAG48ez06TZft=[e-mail]
> Reported-by: xingwei lee <[e-mail]>
> Closes: https://lore.kernel.org/bpf/CABOYnLynjBoFZOf3Z4BhaZkc5hx_kHfsjiW+UWLoB=[e-mail]
> Signed-off-by: Hou Tao <[e-mail]>

Acked-by: Thomas Gleixner <[e-mail]>

Patch

diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c
index 6993f026adec9..42115ac079cfe 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 +17,14 @@  bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
 	if (vaddr < TASK_SIZE_MAX + PAGE_SIZE)
 		return false;
 
+	/*
+	 * Reading from the vsyscall page may cause an unhandled fault in
+	 * certain cases.  Though it is at an address above TASK_SIZE_MAX, it is
+	 * usually considered as a user space address.
+	 */
+	if (is_vsyscall_vaddr(vaddr))
+		return false;
+
 	/*
 	 * Allow everything during early boot before 'x86_virt_bits'
 	 * is initialized.  Needed for instruction decoding in early