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 overflows and
turns into a userspace address, the BPF program might read that memory if
the user has mapped it.
There are architectural features that prevent the kernel from accessing
userspace memory, like Privileged Access Never (PAN) on ARM64,
Supervisor Mode Access Prevention (SMAP) on x86-64, Supervisor User
Memory access (SUM) on RISC-V, etc. But BPF should not rely on the
existence of these features.
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 analyse the same program after it has gone through the JITs of
X86-64, 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 Verifier's Instrumentation
(upstream) (This patch)
--------------------- --------------------------
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 $0x800000000000,%r11 f: mov %rdi,%r10
19: cmp %r11,%rdi 12: shr $0x20,%r10
1c: jb 0x000000000000002a 16: cmp $0x8000,%r10
1e: mov %rdi,%r11 1d: jbe 0x0000000000000025
21: add $0x0,%r11 /--> 1f: mov 0x0(%rdi),%rdi
28: jae 0x000000000000002e / 23: jmp 0x0000000000000027
2a: xor %edi,%edi / 25: xor %edi,%edi
2c: jmp 0x0000000000000032 / /------------------------------------
2e: mov 0x0(%rdi),%rdi ---/ / 27: xor %eax,%eax
---------------------------------/ 29: leave
32: xor %eax,%eax 2a: ret
34: leave
35: ret
The x86-64 JIT already emits some instructions to protect against user
memory access. The implementation in this patch leads to a smaller
number of instructions being emitted. In the worst case the JIT will
emit 9 extra instructions and this patch decreases it to 7.
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]>
---
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.
---
arch/x86/net/bpf_jit_comp.c | 72 +++++--------------------------------
include/linux/filter.h | 1 +
kernel/bpf/core.c | 9 +++++
kernel/bpf/verifier.c | 30 ++++++++++++++++
4 files changed, 48 insertions(+), 64 deletions(-)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index a7ba8e178645..d3268b7a562d 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1327,7 +1327,6 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
u8 b2 = 0, b3 = 0;
u8 *start_of_ldx;
s64 jmp_offset;
- s16 insn_off;
u8 jmp_cond;
u8 *func;
int nops;
@@ -1802,78 +1801,18 @@ st: if (is_imm8(insn->off))
case BPF_LDX | BPF_PROBE_MEMSX | BPF_B:
case BPF_LDX | BPF_PROBE_MEMSX | BPF_H:
case BPF_LDX | BPF_PROBE_MEMSX | BPF_W:
- insn_off = 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
- */
-
- u64 limit = TASK_SIZE_MAX + PAGE_SIZE;
- 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 r11, limit */
- EMIT2(add_1mod(0x48, AUX_REG), add_1reg(0xB8, AUX_REG));
- EMIT((u32)limit, 4);
- EMIT(limit >> 32, 4);
-
- 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);
- }
-
- /* cmp src_reg, r11 */
- maybe_emit_mod(&prog, src_reg, AUX_REG, true);
- EMIT2(0x39, add_2reg(0xC0, src_reg, AUX_REG));
-
- /* if unsigned '>=', goto load */
- EMIT2(X86_JAE, 0);
- end_of_jmp = prog;
-
- /* xor dst_reg, dst_reg */
- emit_mov_imm32(&prog, false, dst_reg, 0);
- /* jmp byte_after_ldx */
- EMIT2(0xEB, 0);
-
- /* populate jmp_offset for JAE above to jump to start_of_ldx */
- start_of_ldx = prog;
- end_of_jmp[-1] = start_of_ldx - end_of_jmp;
- }
+ start_of_ldx = prog;
if (BPF_MODE(insn->code) == BPF_PROBE_MEMSX ||
BPF_MODE(insn->code) == BPF_MEMSX)
- emit_ldsx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn_off);
+ emit_ldsx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
else
- emit_ldx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn_off);
+ emit_ldx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
if (BPF_MODE(insn->code) == BPF_PROBE_MEM ||
BPF_MODE(insn->code) == BPF_PROBE_MEMSX) {
struct exception_table_entry *ex;
u8 *_insn = image + proglen + (start_of_ldx - temp);
s64 delta;
- /* 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;
@@ -3476,3 +3415,8 @@ bool bpf_jit_supports_ptr_xchg(void)
{
return true;
}
+
+u64 bpf_arch_uaddress_limit(void)
+{
+ return max(TASK_SIZE_MAX + PAGE_SIZE, VSYSCALL_ADDR);
+}
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 1dd3b99d1bb9..e125940a3f79 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19660,6 +19660,36 @@ 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
On Sun, Mar 24, 2024 at 11:54 AM Puranjay Mohan <[email protected]> wrote:
>
> +u64 bpf_arch_uaddress_limit(void)
> +{
> + return max(TASK_SIZE_MAX + PAGE_SIZE, VSYSCALL_ADDR);
This is broken. See my other email.
Sadly you didn't test it.
Alexei Starovoitov <[email protected]> writes:
> On Sun, Mar 24, 2024 at 11:54 AM Puranjay Mohan <puranjay12@gmailcom> wrote:
>>
>> +u64 bpf_arch_uaddress_limit(void)
>> +{
>> + return max(TASK_SIZE_MAX + PAGE_SIZE, VSYSCALL_ADDR);
>
> This is broken. See my other email.
> Sadly you didn't test it.
Yes, sorry for this. I was relying on the CI for the test this time as
thought it would work. I just realised this would reject all addresses!
Given that the current x86-64 JIT just tests for TASK_SIZE_MAX +
PAGE_SIZE can we go ahead with this and later fix it for VSYSCALL_ADDR
as it is specific for x86-64?
Also, I will spend some time figuring out the best way to do this, there
should be some mathematical trick somewhere.
On Sun, Mar 24, 2024 at 12:35 PM Puranjay Mohan <[email protected]> wrote:
>
> Alexei Starovoitov <[email protected]> writes:
>
> > On Sun, Mar 24, 2024 at 11:54 AM Puranjay Mohan <[email protected]> wrote:
> >>
> >> +u64 bpf_arch_uaddress_limit(void)
> >> +{
> >> + return max(TASK_SIZE_MAX + PAGE_SIZE, VSYSCALL_ADDR);
> >
> > This is broken. See my other email.
> > Sadly you didn't test it.
>
> Yes, sorry for this. I was relying on the CI for the test this time as
> thought it would work. I just realised this would reject all addresses!
>
> Given that the current x86-64 JIT just tests for TASK_SIZE_MAX +
> PAGE_SIZE can we go ahead with this and later fix it for VSYSCALL_ADDR
> as it is specific for x86-64?
This patch deletes a bunch of code from arch/x86/net/bpf_jit_comp.c.
If you're proposing to deal with vsyscall check in JIT then
very similar code in the same spot will be re-introduced.
So we should address all issues at once to avoid code churn.
> Also, I will spend some time figuring out the best way to do this, there
> should be some mathematical trick somewhere.
Let's think of it right now.
For the next patch you can reduce the cc list to avoid spamming all
mailing lists.
bpf@vger plus JIT folks for affected arches will be enough.