bpf: properly verify tail call behavior #10241
Closed
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Author
|
Upstream branch: f8c67d8 |
kernel-patches-daemon-bpf
bot
force-pushed
the
bpf-next_base
branch
from
b6555d5 to
910d3ce
Compare
A successful ebpf tail call does not return to the caller, but to the caller-of-the-caller, often just finishing the ebpf program altogether. Any restrictions that the verifier needs to take into account - notably the fact that the tail call might have modified packet pointers - are to be checked on the caller-of-the-caller. Checking it on the caller made the verifier refuse perfectly fine programs that would use the packet pointers after a tail call, which is no problem as this code is only executed if the tail call was unsuccessful, i.e. nothing happened. This patch simulates the behavior of a tail call in the verifier. A conditional jump to the code after the tail call is added for the case of an unsucessful tail call, and a return to the caller is simulated for a successful tail call. For the successful case we assume that the tail call returns an int, as tail calls are currently only allowed in functions that return and int. We always assume that the tail call modified the packet pointers, as we do not know what the tail call did. For the unsuccessful case we know nothing happened, so we do not need to add new constraints. This approach also allows to check other problems that may occur with tail calls, namely we are now able to check that precision is properly propagated into subprograms using tail calls, as well as checking the live slots in such a subprogram. Fixes: 1a4607f ("bpf: consider that tail calls invalidate packet pointers") Link: https://lore.kernel.org/bpf/[email protected]/ Signed-off-by: Martin Teichmann <[email protected]>
Four tests are added: - invalidate_pkt_pointers_by_tail_call checks that one can use the packet pointer after a tail call. This was originally possible and also poses not problems, but was made impossible by 1a4607f. - invalidate_pkt_pointers_by_static_tail_call tests a corner case found by Eduard Zingerman during the discussion of the original fix, which was broken in that fix. - subprog_result_tail_call tests that precision propagation works correctly across tail calls. This did not work before. - caller_stack_write_tail_call tests that the live stack is correctly tracked for a tail call, again a corner case found by Eduard Zingerman. Signed-off-by: Martin Teichmann <[email protected]>
Author
|
Upstream branch: 026bcf9 |
kernel-patches-daemon-bpf
bot
force-pushed
the
series/1021656=>bpf-next
branch
from
a85c5e1 to
0a9c10a
Compare
Author
|
At least one diff in series https://patchwork.kernel.org/project/netdevbpf/list/?series=1021656 expired. Closing PR. |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Pull request for series with
subject: bpf: properly verify tail call behavior
version: 4
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1021656