On Wed, Apr 6, 2022 at 5:12 PM Nikolay Borisov <nborisov@xxxxxxxx> wrote:
>
> Hello,
>
> I get a validator error with the following function:
>
> #define MAX_PERCPU_BUFSIZE (1<<15)
> #define PATH_MAX 4096
> #define MAX_PATH_COMPONENTS 20
> #define IDX(x) ((x) & (MAX_PERCPU_BUFSIZE-1))
>
> struct buf_t {
> u8 buf[MAX_PERCPU_BUFSIZE];
> };
>
> struct {
> __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
> __uint(max_entries, 1);
> __type(key, u32);
> __type(value, struct buf_t);
> } buf_map SEC(".maps");
>
> static __always_inline u32 get_dentry_path_str(struct dentry* dentry,
> struct buf_t *string_p)
> {
> const char slash = '/';
> const int zero = 0;
>
> u32 buf_off = MAX_PERCPU_BUFSIZE - 1;
>
> #pragma unroll
> for (int i = 0; i < MAX_PATH_COMPONENTS; i++) {
> struct dentry *d_parent = BPF_CORE_READ(dentry, d_parent);
> if (dentry == d_parent) {
> break;
> }
> // Add this dentry name to path
> struct qstr d_name = BPF_CORE_READ(dentry, d_name);
> // Ensure path is no longer than PATH_MAX-1 and copy the terminating NULL
> unsigned int len = (d_name.len+1) & (PATH_MAX-1);
> // Start writing from the end of the buffer
> unsigned int off = buf_off - len;
> // Is string buffer big enough for dentry name?
> int sz = 0;
> // verify no wrap occurred
> if (off <= buf_off)
> sz = bpf_probe_read_kernel_str(&string_p->buf[IDX(off)], len, (void *)d_name.name);
> else
> break;
>
> if (sz > 1) {
> buf_off -= 1; // replace null byte termination with slash sign
> bpf_probe_read(&(string_p->buf[IDX(buf_off)]), 1, &slash);
> buf_off -= sz - 1;
> } else {
> // If sz is 0 or 1 we have an error (path can't be null nor an empty string)
> break;
> }
> dentry = d_parent;
> }
>
> // Add leading slash
> buf_off -= 1;
> bpf_probe_read(&(string_p->buf[IDX(buf_off)]), 1, &slash);
> // Null terminate the path string, this replaces the final / with a null
> // char
> bpf_probe_read(&(string_p->buf[MAX_PERCPU_BUFSIZE-1]), 1, &zero);
> return buf_off;
> }
>
> Here are the last couple of instructions where off is being calculated.
>
> ; unsigned int len = (d_name.len+1) & (PATH_MAX-1);
> 54: (57) r9 &= 4095 ; R9_w=inv(id=0,umax_value=4095,var_off=(0x0; 0xfff))
> ; unsigned int off = buf_off - len;
> 55: (bf) r8 = r9 ; R8_w=inv(id=4,umax_value=4095,var_off=(0x0; 0xfff)) R9_w=inv(id=4,umax_value=4095,var_off=(0x0; 0xfff))
> 56: (a7) r8 ^= 32767 ; R8_w=inv(id=0,umin_value=28672,umax_value=32767,var_off=(0x7000; 0xfff))
> ; sz = bpf_probe_read_kernel_str(&string_p->buf[IDX(off)], len, (void *)d_name.name);
> 57: (79) r6 = *(u64 *)(r10 -72) ; R6_w=map_value(id=0,off=0,ks=4,vs=32768,imm=0) R10=fp0
> 58: (0f) r6 += r8 ; R6_w=map_value(id=0,off=0,ks=4,vs=32768,umin_value=28672,umax_value=32767,var_off=(0x7000; 0xfff)) R8_w=invP(id=0,umin_value=28672,umax_value=32767,var_off=(0x7000; 0xfff))
> ; sz = bpf_probe_read_kernel_str(&string_p->buf[IDX(off)], len, (void *)d_name.name);
> 59: (79) r3 = *(u64 *)(r1 +8) ; R1_w=fp-24 R3_w=inv(id=0)
> ; sz = bpf_probe_read_kernel_str(&string_p->buf[IDX(off)], len, (void *)d_name.name);
> 60: (bf) r1 = r6 ; R1_w=map_value(id=0,off=0,ks=4,vs=32768,umin_value=28672,umax_value=32767,var_off=(0x7000; 0xfff)) R6_w=map_value(id=0,off=0,ks=4,vs=32768,umin_value=28672,umax_value=32767,var_off=(0x7000; 0xfff))
> 61: (bf) r2 = r9 ; R2_w=inv(id=4,umax_value=4095,var_off=(0x0; 0xfff)) R9_w=inv(id=4,umax_value=4095,var_off=(0x0; 0xfff))
> 62: (85) call bpf_probe_read_kernel_str#115
> invalid access to map value, value_size=32768 off=32767 size=4095
> R1 max value is outside of the allowed memory range
>
>
> From what I gathered it seems that in the bpf_probe_read_kernel_str the validator is not
> able to prove that off+len is always going to be at most MAX_PERCPU_BUFSIZE - 1 which is well within
> the bounds of the buffer. My logic goes as following:
>
> buf_off is first set to 32767, we get the first dentry and calculate its len + null terminating char which is going to be at most
> 4095, afterwards 'off' is being calculated as buf_off - len and finally access to the percpu array is performed via IDX(off) which guarantees the access is
> bounded by MAX_PERCPU_BUFSIZE - 1.
IDX(off) is bounded, but verifier needs to be sure that `off + len`
never goes beyond map value. so you should make sure and prove off <=
MAX_PERCPU_BUFSIZE - PATH_MAX. Verifier actually caught a real bug for
you.
>
> This code was originally based off https://github.com/aquasecurity/tracee/blob/main/pkg/ebpf/c/tracee.bpf.c#L1721-L1777 however it seems
> that the way tracee author work around this is to simply start from the middle of the buffer, i.e set buf_off initially to MAX_PERCPU_BUFSIZE>>1 and adjust the
> array accesses accordingly when doing the masking.
