Error validating array access

* Error validating array access
@ 2022-04-06 19:51 Nikolay Borisov
  2022-04-08 22:58 ` Andrii Nakryiko
  0 siblings, 1 reply; 8+ messages in thread
From: Nikolay Borisov @ 2022-04-06 19:51 UTC (permalink / raw)
  To: bpf

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.

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.

^ permalink raw reply	[flat|nested] 8+ messages in thread