V5: https://lore.kernel.org/bpf/[email protected]/
Changes in V6:
- Disable the verifier's instrumentation in x86-64 and update the JIT to
take care of vsyscall page in addition to userspace addresses.
- Update bpf_testmod to test for vsyscall addresses.
V4: https://lore.kernel.org/bpf/[email protected]/
Changes in V5:
- Use TASK_SIZE_MAX + PAGE_SIZE, VSYSCALL_ADDR as userspace boundary in
x86-64 JIT.
- Added Acked-by: Ilya Leoshkevich <[email protected]>
V3: https://lore.kernel.org/bpf/[email protected]/
Changes in V4:
- Disable this feature on architectures that don't define
CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE.
- By doing the above, we don't need anything explicitly for s390x.
V2: https://lore.kernel.org/bpf/[email protected]/
Changes in V3:
- Return 0 from bpf_arch_uaddress_limit() in disabled case because it
returns u64.
- Modify the check in verifier to no do instrumentation when uaddress_limit
is 0.
V1: https://lore.kernel.org/bpf/[email protected]/
Changes in V2:
- Disable this feature on s390x.
With BPF_PROBE_MEM, BPF allows de-referencing an untrusted pointer. To
thwart invalid memory accesses, the JITs add an exception table entry for
all such accesses. But in case the src_reg + offset is a userspace address,
the BPF program might read that memory if the user has mapped it.
x86-64 JIT already instruments the BPF_PROBE_MEM based loads with checks to
skip loads from userspace addresses, but is doesn't check for vsyscall page
because it falls in the kernel address space but is considered a userspace
page. The second patch in this series fixes the x86-64 JIT to also skip
loads from the vsyscall page. The last patch updates the bpf_testmod so
this address can be checked as part of the selftests.
Other architectures don't have the complexity of the vsyscall address and
just need to skip loads from the userspace. To make this more scalable and
robust, the verifier is updated in the first patch to instrument
BPF_PROBE_MEM to skip loads from the userspace addresses.
Puranjay Mohan (3):
bpf: verifier: prevent userspace memory access
bpf, x86: Fix PROBE_MEM runtime load check
selftests/bpf: Test PROBE_MEM of VSYSCALL_ADDR on x86-64
arch/x86/net/bpf_jit_comp.c | 63 +++++++++----------
include/linux/filter.h | 1 +
kernel/bpf/core.c | 9 +++
kernel/bpf/verifier.c | 30 +++++++++
.../selftests/bpf/bpf_testmod/bpf_testmod.c | 3 +
5 files changed, 74 insertions(+), 32 deletions(-)
--
2.40.1
From: Puranjay Mohan <[email protected]>
With BPF_PROBE_MEM, BPF allows de-referencing an untrusted pointer. To
thwart invalid memory accesses, the JITs add an exception table entry
for all such accesses. But in case the src_reg + offset is a userspace
address, the BPF program might read that memory if the user has
mapped it.
Make the verifier add guard instructions around such memory accesses and
skip the load if the address falls into the userspace region.
The JITs need to implement bpf_arch_uaddress_limit() to define where
the userspace addresses end for that architecture or TASK_SIZE is taken
as default.
The implementation is as follows:
REG_AX = SRC_REG
if(offset)
REG_AX += offset;
REG_AX >>= 32;
if (REG_AX <= (uaddress_limit >> 32))
DST_REG = 0;
else
DST_REG = *(size *)(SRC_REG + offset);
Comparing just the upper 32 bits of the load address with the upper
32 bits of uaddress_limit implies that the values are being aligned down
to a 4GB boundary before comparison.
The above means that all loads with address <= uaddress_limit + 4GB are
skipped. This is acceptable because there is a large hole (much larger
than 4GB) between userspace and kernel space memory, therefore a
correctly functioning BPF program should not access this 4GB memory
above the userspace.
Let's analyze what this patch does to the following fentry program
dereferencing an untrusted pointer:
SEC("fentry/tcp_v4_connect")
int BPF_PROG(fentry_tcp_v4_connect, struct sock *sk)
{
*(volatile long *)sk;
return 0;
}
BPF Program before | BPF Program after
------------------ | -----------------
0: (79) r1 = *(u64 *)(r1 +0) 0: (79) r1 = *(u64 *)(r1 +0)
-----------------------------------------------------------------------
1: (79) r1 = *(u64 *)(r1 +0) --\ 1: (bf) r11 = r1
----------------------------\ \ 2: (77) r11 >>= 32
2: (b7) r0 = 0 \ \ 3: (b5) if r11 <= 0x8000 goto pc+2
3: (95) exit \ \-> 4: (79) r1 = *(u64 *)(r1 +0)
\ 5: (05) goto pc+1
\ 6: (b7) r1 = 0
\--------------------------------------
7: (b7) r0 = 0
8: (95) exit
As you can see from above, in the best case (off=0), 5 extra instructions
are emitted.
Now, we analyze the same program after it has gone through the JITs of
ARM64 and RISC-V architectures. We follow the single load instruction
that has the untrusted pointer and see what instrumentation has been
added around it.
x86-64 JIT
==========
JIT's Instrumentation
(upstream)
---------------------
0: nopl 0x0(%rax,%rax,1)
5: xchg %ax,%ax
7: push %rbp
8: mov %rsp,%rbp
b: mov 0x0(%rdi),%rdi
---------------------------------
f: movabs $0x800000000000,%r11
19: cmp %r11,%rdi
1c: jb 0x000000000000002a
1e: mov %rdi,%r11
21: add $0x0,%r11
28: jae 0x000000000000002e
2a: xor %edi,%edi
2c: jmp 0x0000000000000032
2e: mov 0x0(%rdi),%rdi
---------------------------------
32: xor %eax,%eax
34: leave
35: ret
The x86-64 JIT already emits some instructions to protect against user
memory access. This patch doesn't make any changes for the x86-64 JIT.
ARM64 JIT
=========
No Intrumentation Verifier's Instrumentation
(upstream) (This patch)
----------------- --------------------------
0: add x9, x30, #0x0 0: add x9, x30, #0x0
4: nop 4: nop
8: paciasp 8: paciasp
c: stp x29, x30, [sp, #-16]! c: stp x29, x30, [sp, #-16]!
10: mov x29, sp 10: mov x29, sp
14: stp x19, x20, [sp, #-16]! 14: stp x19, x20, [sp, #-16]!
18: stp x21, x22, [sp, #-16]! 18: stp x21, x22, [sp, #-16]!
1c: stp x25, x26, [sp, #-16]! 1c: stp x25, x26, [sp, #-16]!
20: stp x27, x28, [sp, #-16]! 20: stp x27, x28, [sp, #-16]!
24: mov x25, sp 24: mov x25, sp
28: mov x26, #0x0 28: mov x26, #0x0
2c: sub x27, x25, #0x0 2c: sub x27, x25, #0x0
30: sub sp, sp, #0x0 30: sub sp, sp, #0x0
34: ldr x0, [x0] 34: ldr x0, [x0]
--------------------------------------------------------------------------------
38: ldr x0, [x0] ----------\ 38: add x9, x0, #0x0
-----------------------------------\\ 3c: lsr x9, x9, #32
3c: mov x7, #0x0 \\ 40: cmp x9, #0x10, lsl #12
40: mov sp, sp \\ 44: b.ls 0x0000000000000050
44: ldp x27, x28, [sp], #16 \\--> 48: ldr x0, [x0]
48: ldp x25, x26, [sp], #16 \ 4c: b 0x0000000000000054
4c: ldp x21, x22, [sp], #16 \ 50: mov x0, #0x0
50: ldp x19, x20, [sp], #16 \---------------------------------------
54: ldp x29, x30, [sp], #16 54: mov x7, #0x0
58: add x0, x7, #0x0 58: mov sp, sp
5c: autiasp 5c: ldp x27, x28, [sp], #16
60: ret 60: ldp x25, x26, [sp], #16
64: nop 64: ldp x21, x22, [sp], #16
68: ldr x10, 0x0000000000000070 68: ldp x19, x20, [sp], #16
6c: br x10 6c: ldp x29, x30, [sp], #16
70: add x0, x7, #0x0
74: autiasp
78: ret
7c: nop
80: ldr x10, 0x0000000000000088
84: br x10
There are 6 extra instructions added in ARM64 in the best case. This will
become 7 in the worst case (off != 0).
RISC-V JIT (RISCV_ISA_C Disabled)
==========
No Intrumentation Verifier's Instrumentation
(upstream) (This patch)
----------------- --------------------------
0: nop 0: nop
4: nop 4: nop
8: li a6, 33 8: li a6, 33
c: addi sp, sp, -16 c: addi sp, sp, -16
10: sd s0, 8(sp) 10: sd s0, 8(sp)
14: addi s0, sp, 16 14: addi s0, sp, 16
18: ld a0, 0(a0) 18: ld a0, 0(a0)
---------------------------------------------------------------
1c: ld a0, 0(a0) --\ 1c: mv t0, a0
--------------------------\ \ 20: srli t0, t0, 32
20: li a5, 0 \ \ 24: lui t1, 4096
24: ld s0, 8(sp) \ \ 28: sext.w t1, t1
28: addi sp, sp, 16 \ \ 2c: bgeu t1, t0, 12
2c: sext.w a0, a5 \ \--> 30: ld a0, 0(a0)
30: ret \ 34: j 8
\ 38: li a0, 0
\------------------------------
3c: li a5, 0
40: ld s0, 8(sp)
44: addi sp, sp, 16
48: sext.w a0, a5
4c: ret
There are 7 extra instructions added in RISC-V.
Fixes: 800834285361 ("bpf, arm64: Add BPF exception tables")
Reported-by: Breno Leitao <[email protected]>
Suggested-by: Alexei Starovoitov <[email protected]>
Acked-by: Ilya Leoshkevich <[email protected]>
Signed-off-by: Puranjay Mohan <[email protected]>
---
arch/x86/net/bpf_jit_comp.c | 6 ++++++
include/linux/filter.h | 1 +
kernel/bpf/core.c | 9 +++++++++
kernel/bpf/verifier.c | 30 ++++++++++++++++++++++++++++++
4 files changed, 46 insertions(+)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index df5fac428408..b520b66ad14b 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -3473,3 +3473,9 @@ bool bpf_jit_supports_ptr_xchg(void)
{
return true;
}
+
+/* x86-64 JIT emits its own code to filter user addresses so return 0 here */
+u64 bpf_arch_uaddress_limit(void)
+{
+ return 0;
+}
diff --git a/include/linux/filter.h b/include/linux/filter.h
index c99bc3df2d28..219ee7a76874 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -963,6 +963,7 @@ bool bpf_jit_supports_far_kfunc_call(void);
bool bpf_jit_supports_exceptions(void);
bool bpf_jit_supports_ptr_xchg(void);
bool bpf_jit_supports_arena(void);
+u64 bpf_arch_uaddress_limit(void);
void arch_bpf_stack_walk(bool (*consume_fn)(void *cookie, u64 ip, u64 sp, u64 bp), void *cookie);
bool bpf_helper_changes_pkt_data(void *func);
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 696bc55de8e8..1ea5ce5bb599 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2942,6 +2942,15 @@ bool __weak bpf_jit_supports_arena(void)
return false;
}
+u64 __weak bpf_arch_uaddress_limit(void)
+{
+#if defined(CONFIG_64BIT) && defined(CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE)
+ return TASK_SIZE;
+#else
+ return 0;
+#endif
+}
+
/* Return TRUE if the JIT backend satisfies the following two conditions:
* 1) JIT backend supports atomic_xchg() on pointer-sized words.
* 2) Under the specific arch, the implementation of xchg() is the same
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index aa24beb65393..cb7ad1f795e1 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19675,6 +19675,36 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
goto next_insn;
}
+ /* Make it impossible to de-reference a userspace address */
+ if (BPF_CLASS(insn->code) == BPF_LDX &&
+ (BPF_MODE(insn->code) == BPF_PROBE_MEM ||
+ BPF_MODE(insn->code) == BPF_PROBE_MEMSX)) {
+ struct bpf_insn *patch = &insn_buf[0];
+ u64 uaddress_limit = bpf_arch_uaddress_limit();
+
+ if (!uaddress_limit)
+ goto next_insn;
+
+ *patch++ = BPF_MOV64_REG(BPF_REG_AX, insn->src_reg);
+ if (insn->off)
+ *patch++ = BPF_ALU64_IMM(BPF_ADD, BPF_REG_AX, insn->off);
+ *patch++ = BPF_ALU64_IMM(BPF_RSH, BPF_REG_AX, 32);
+ *patch++ = BPF_JMP_IMM(BPF_JLE, BPF_REG_AX, uaddress_limit >> 32, 2);
+ *patch++ = *insn;
+ *patch++ = BPF_JMP_IMM(BPF_JA, 0, 0, 1);
+ *patch++ = BPF_MOV64_IMM(insn->dst_reg, 0);
+
+ cnt = patch - insn_buf;
+ new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
+ if (!new_prog)
+ return -ENOMEM;
+
+ delta += cnt - 1;
+ env->prog = prog = new_prog;
+ insn = new_prog->insnsi + i + delta;
+ goto next_insn;
+ }
+
/* Implement LD_ABS and LD_IND with a rewrite, if supported by the program type. */
if (BPF_CLASS(insn->code) == BPF_LD &&
(BPF_MODE(insn->code) == BPF_ABS ||
--
2.40.1
The vsyscall is a legacy API for fast execution of system calls. It maps
a page at address VSYSCALL_ADDR into the userspace program. This address
is in the top 10MB of the address space:
ffffffffff600000 - ffffffffff600fff | 4 kB | legacy vsyscall ABI
The last commit fixes the x86-64 BPF JIT to skip accessing addresses in
this memory region. Add this address to bpf_testmod_return_ptr() so we
can make sure that it is fixed.
After this change and without the previous commit, subprogs_extable
selftest will crash the kernel.
Signed-off-by: Puranjay Mohan <[email protected]>
---
tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
index 39ad96a18123..edcd26106557 100644
--- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
+++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
@@ -205,6 +205,9 @@ __weak noinline struct file *bpf_testmod_return_ptr(int arg)
case 5: return (void *)~(1ull << 30); /* trigger extable */
case 6: return &f; /* valid addr */
case 7: return (void *)((long)&f | 1); /* kernel tricks */
+#ifdef CONFIG_X86_64
+ case 8: return (void *)VSYSCALL_ADDR; /* vsyscall page address */
+#endif
default: return NULL;
}
}
--
2.40.1
When a load is marked PROBE_MEM - e.g. due to PTR_UNTRUSTED access - the
address being loaded from is not necessarily valid. The BPF jit sets up
exception handlers for each such load which catch page faults and 0 out
the destination register.
If the address for the load is outside kernel address space, the load
will escape the exception handling and crash the kernel. To prevent this
from happening, the emits some instruction to verify that addr is > end
of userspace addresses.
x86 has a legacy vsyscall ABI where a page at address 0xffffffffff600000
is mapped with user accessible permissions. The addresses in this page
are considered userspace addresses by the fault handler. Therefore, a
BPF program accessing this page will crash the kernel.
This patch fixes the runtime checks to also check that the PROBE_MEM
address is below VSYSCALL_ADDR.
Example BPF program:
SEC("fentry/tcp_v4_connect")
int BPF_PROG(fentry_tcp_v4_connect, struct sock *sk)
{
*(volatile unsigned long *)&sk->sk_tsq_flags;
return 0;
}
BPF Assembly:
0: (79) r1 = *(u64 *)(r1 +0)
1: (79) r1 = *(u64 *)(r1 +344)
2: (b7) r0 = 0
3: (95) exit
x86-64 JIT
==========
BEFORE AFTER
------ -----
0: nopl 0x0(%rax,%rax,1) 0: nopl 0x0(%rax,%rax,1)
5: xchg %ax,%ax 5: xchg %ax,%ax
7: push %rbp 7: push %rbp
8: mov %rsp,%rbp 8: mov %rsp,%rbp
b: mov 0x0(%rdi),%rdi b: mov 0x0(%rdi),%rdi
-------------------------------------------------------------------------------
f: movabs $0x100000000000000,%r11 f: movabs $0xffffffffff600000,%r10
19: add $0x2a0,%rdi 19: mov %rdi,%r11
20: cmp %r11,%rdi 1c: add $0x2a0,%r11
23: jae 0x0000000000000029 23: sub %r10,%r11
25: xor %edi,%edi 26: movabs $0x100000000a00000,%r10
27: jmp 0x000000000000002d 30: cmp %r10,%r11
29: mov 0x0(%rdi),%rdi 33: ja 0x0000000000000039
--------------------------------\ 35: xor %edi,%edi
2d: xor %eax,%eax \ 37: jmp 0x0000000000000040
2f: leave \ 39: mov 0x2a0(%rdi),%rdi
30: ret \--------------------------------------------
40: xor %eax,%eax
42: leave
43: ret
Signed-off-by: Puranjay Mohan <[email protected]>
---
arch/x86/net/bpf_jit_comp.c | 57 ++++++++++++++++---------------------
1 file changed, 25 insertions(+), 32 deletions(-)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index b520b66ad14b..59cbc94b6e69 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1807,36 +1807,41 @@ st: if (is_imm8(insn->off))
if (BPF_MODE(insn->code) == BPF_PROBE_MEM ||
BPF_MODE(insn->code) == BPF_PROBE_MEMSX) {
/* Conservatively check that src_reg + insn->off is a kernel address:
- * src_reg + insn->off >= TASK_SIZE_MAX + PAGE_SIZE
- * src_reg is used as scratch for src_reg += insn->off and restored
- * after emit_ldx if necessary
+ * src_reg + insn->off > TASK_SIZE_MAX + PAGE_SIZE
+ * and
+ * src_reg + insn->off < VSYSCALL_ADDR
*/
- u64 limit = TASK_SIZE_MAX + PAGE_SIZE;
+ u64 limit = TASK_SIZE_MAX + PAGE_SIZE - VSYSCALL_ADDR;
u8 *end_of_jmp;
- /* At end of these emitted checks, insn->off will have been added
- * to src_reg, so no need to do relative load with insn->off offset
- */
- insn_off = 0;
+ /* movabsq r10, VSYSCALL_ADDR */
+ emit_mov_imm64(&prog, BPF_REG_AX, (long)VSYSCALL_ADDR >> 32,
+ (u32)(long)VSYSCALL_ADDR);
- /* movabsq r11, limit */
- EMIT2(add_1mod(0x48, AUX_REG), add_1reg(0xB8, AUX_REG));
- EMIT((u32)limit, 4);
- EMIT(limit >> 32, 4);
+ /* mov src_reg, r11 */
+ EMIT_mov(AUX_REG, src_reg);
if (insn->off) {
- /* add src_reg, insn->off */
- maybe_emit_1mod(&prog, src_reg, true);
- EMIT2_off32(0x81, add_1reg(0xC0, src_reg), insn->off);
+ /* add r11, insn->off */
+ maybe_emit_1mod(&prog, AUX_REG, true);
+ EMIT2_off32(0x81, add_1reg(0xC0, AUX_REG), insn->off);
}
- /* cmp src_reg, r11 */
- maybe_emit_mod(&prog, src_reg, AUX_REG, true);
- EMIT2(0x39, add_2reg(0xC0, src_reg, AUX_REG));
+ /* sub r11, r10 */
+ maybe_emit_mod(&prog, AUX_REG, BPF_REG_AX, true);
+ EMIT2(0x29, add_2reg(0xC0, AUX_REG, BPF_REG_AX));
+
+ /* movabsq r10, limit */
+ emit_mov_imm64(&prog, BPF_REG_AX, (long)limit >> 32,
+ (u32)(long)limit);
+
+ /* cmp r10, r11 */
+ maybe_emit_mod(&prog, AUX_REG, BPF_REG_AX, true);
+ EMIT2(0x39, add_2reg(0xC0, AUX_REG, BPF_REG_AX));
- /* if unsigned '>=', goto load */
- EMIT2(X86_JAE, 0);
+ /* if unsigned '>', goto load */
+ EMIT2(X86_JA, 0);
end_of_jmp = prog;
/* xor dst_reg, dst_reg */
@@ -1862,18 +1867,6 @@ st: if (is_imm8(insn->off))
/* populate jmp_offset for JMP above */
start_of_ldx[-1] = prog - start_of_ldx;
- if (insn->off && src_reg != dst_reg) {
- /* sub src_reg, insn->off
- * Restore src_reg after "add src_reg, insn->off" in prev
- * if statement. But if src_reg == dst_reg, emit_ldx
- * above already clobbered src_reg, so no need to restore.
- * If add src_reg, insn->off was unnecessary, no need to
- * restore either.
- */
- maybe_emit_1mod(&prog, src_reg, true);
- EMIT2_off32(0x81, add_1reg(0xE8, src_reg), insn->off);
- }
-
if (!bpf_prog->aux->extable)
break;
--
2.40.1
Hello:
This series was applied to bpf/bpf.git (master)
by Alexei Starovoitov <[email protected]>:
On Wed, 24 Apr 2024 10:02:07 +0000 you wrote:
> V5: https://lore.kernel.org/bpf/[email protected]/
> Changes in V6:
> - Disable the verifier's instrumentation in x86-64 and update the JIT to
> take care of vsyscall page in addition to userspace addresses.
> - Update bpf_testmod to test for vsyscall addresses.
>
> V4: https://lore.kernel.org/bpf/[email protected]/
> Changes in V5:
> - Use TASK_SIZE_MAX + PAGE_SIZE, VSYSCALL_ADDR as userspace boundary in
> x86-64 JIT.
> - Added Acked-by: Ilya Leoshkevich <[email protected]>
>
> [...]
Here is the summary with links:
- [bpf,v6,1/3] bpf: verifier: prevent userspace memory access
https://git.kernel.org/bpf/bpf/c/66e13b615a0c
- [bpf,v6,2/3] bpf, x86: Fix PROBE_MEM runtime load check
https://git.kernel.org/bpf/bpf/c/b599d7d26d6a
- [bpf,v6,3/3] selftests/bpf: Test PROBE_MEM of VSYSCALL_ADDR on x86-64
https://git.kernel.org/bpf/bpf/c/7cd6750d9a56
You are awesome, thank you!
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