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A somewhat cumbersome test case sensitive to correct copying of
bpf_verifier_state->loop_entry fields in
verifier.c:copy_verifier_state().
W/o the fix from a previous commit the program is accepted as safe.
1: /* poison block */
2: if (random() != 24) { // assume false branch is placed first
3: i = iter_new();
4: while (iter_next(i));
5: iter_destroy(i);
6: return;
7: }
8:
9: /* dfs_depth block */
10: for (i = 10; i > 0; i--);
11:
12: /* main block */
13: i = iter_new(); // fp[-16]
14: b = -24; // r8
15: for (;;) {
16: if (iter_next(i))
17: break;
18: if (random() == 77) { // assume false branch is placed first
19: *(u64 *)(r10 + b) = 7; // this is not safe when b == -25
20: iter_destroy(i);
21: return;
22: }
23: if (random() == 42) { // assume false branch is placed first
24: b = -25;
25: }
26: }
27: iter_destroy(i);
The goal of this example is to:
(a) poison env->cur_state->loop_entry with a state S,
such that S->branches == 0;
(b) set state S as a loop_entry for all checkpoints in
/* main block */, thus forcing NOT_EXACT states comparisons;
(c) exploit incorrect loop_entry set for checkpoint at line 18
by first creating a checkpoint with b == -24 and then
pruning the state with b == -25 using that checkpoint.
The /* poison block */ is responsible for goal (a).
It forces verifier to first validate some unrelated iterator based
loop, which leads to an update_loop_entry() call in is_state_visited(),
which places checkpoint created at line 4 as env->cur_state->loop_entry.
Starting from line 8, the branch count for that checkpoint is 0.
The /* dfs_depth block */ is responsible for goal (b).
It abuses the fact that update_loop_entry(cur, hdr) only updates
cur->loop_entry when hdr->dfs_depth <= cur->dfs_depth.
After line 12 every state has dfs_depth bigger then dfs_depth of
poisoned env->cur_state->loop_entry. Thus the above condition is never
true for lines 12-27.
The /* main block */ is responsible for goal (c).
Verification proceeds as follows:
- checkpoint {b=-24,i=active} created at line 16;
- jump 18->23 is verified first, jump to 19 pushed to stack;
- jump 23->26 is verified first, jump to 24 pushed to stack;
- checkpoint {b=-24,i=active} created at line 15;
- current state is pruned by checkpoint created at line 16,
this sets branches count for checkpoint at line 15 to 0;
- jump to 24 is popped from stack;
- line 16 is reached in state {b=-25,i=active};
- this is pruned by a previous checkpoint {b=-24,i=active}:
- checkpoint's loop_entry is poisoned and has branch count of 0,
hence states are compared using NOT_EXACT rules;
- b is not marked precise yet.
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250215110411.3236773-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.