CVE-2024-26589

In the Linux kernel, the following vulnerability has been resolved:

bpf: Reject variable offset alu on PTR_TO_FLOW_KEYS

For PTR_TO_FLOW_KEYS, check_flow_keys_access() only uses fixed off

for validation. However, variable offset ptr alu is not prohibited

for this ptr kind. So the variable offset is not checked.

The following prog is accepted:

func#0 @0

0: R1=ctx() R10=fp0

0: (bf) r6 = r1 ; R1=ctx() R6_w=ctx()

1: (79) r7 = *(u64 *)(r6 +144) ; R6_w=ctx() R7_w=flow_keys()

2: (b7) r8 = 1024 ; R8_w=1024

3: (37) r8 /= 1 ; R8_w=scalar()

4: (57) r8 &= 1024 ; R8_w=scalar(smin=smin32=0,

smax=umax=smax32=umax32=1024,var_off=(0x0; 0x400))

5: (0f) r7 += r8

mark_precise: frame0: last_idx 5 first_idx 0 subseq_idx -1

mark_precise: frame0: regs=r8 stack= before 4: (57) r8 &= 1024

mark_precise: frame0: regs=r8 stack= before 3: (37) r8 /= 1

mark_precise: frame0: regs=r8 stack= before 2: (b7) r8 = 1024

6: R7_w=flow_keys(smin=smin32=0,smax=umax=smax32=umax32=1024,var_off

=(0x0; 0x400)) R8_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=1024,

var_off=(0x0; 0x400))

6: (79) r0 = *(u64 *)(r7 +0) ; R0_w=scalar()

7: (95) exit

This prog loads flow_keys to r7, and adds the variable offset r8

to r7, and finally causes out-of-bounds access:

BUG: unable to handle page fault for address: ffffc90014c80038

[...]

Call Trace:

<TASK>

bpf_dispatcher_nop_func include/linux/bpf.h:1231 [inline]

__bpf_prog_run include/linux/filter.h:651 [inline]

bpf_prog_run include/linux/filter.h:658 [inline]

bpf_prog_run_pin_on_cpu include/linux/filter.h:675 [inline]

bpf_flow_dissect+0x15f/0x350 net/core/flow_dissector.c:991

bpf_prog_test_run_flow_dissector+0x39d/0x620 net/bpf/test_run.c:1359

bpf_prog_test_run kernel/bpf/syscall.c:4107 [inline]

__sys_bpf+0xf8f/0x4560 kernel/bpf/syscall.c:5475

__do_sys_bpf kernel/bpf/syscall.c:5561 [inline]

__se_sys_bpf kernel/bpf/syscall.c:5559 [inline]

__x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:5559

do_syscall_x64 arch/x86/entry/common.c:52 [inline]

do_syscall_64+0x3f/0x110 arch/x86/entry/common.c:83

entry_SYSCALL_64_after_hwframe+0x63/0x6b

Fix this by rejecting ptr alu with variable offset on flow_keys.

Applying the patch rejects the program with "R7 pointer arithmetic

on flow_keys prohibited".