lockdep: Fix block chain corruption

Kent reported an occasional KASAN splat in lockdep. Mark then noted:

> I suspect the dodgy access is to chain_block_buckets[-1], which hits the last 4
> bytes of the redzone and gets (incorrectly/misleadingly) attributed to
> nr_large_chain_blocks.

That would mean @size == 0, at which point size_to_bucket() returns -1
and the above happens.

alloc_chain_hlocks() has 'size - req', for the first with the
precondition 'size >= rq', which allows the 0.

This code is trying to split a block, del_chain_block() takes what we
need, and add_chain_block() puts back the remainder, except in the
above case the remainder is 0 sized and things go sideways.

Fixes: 810507fe6fd5 ("locking/lockdep: Reuse freed chain_hlocks entries")
Reported-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Kent Overstreet <kent.overstreet@linux.dev>
Link: https://lkml.kernel.org/r/20231121114126.GH8262@noisy.programming.kicks-ass.net

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index e85b5ad3e206987c1a4d82e81f0386627b01b81e..151bd3de59363a6b67a3a51274fe568bae4b30dd 100644 (file)
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3497,7 +3497,8 @@ static int alloc_chain_hlocks(int req)
                size = chain_block_size(curr);
                if (likely(size >= req)) {
                        del_chain_block(0, size, chain_block_next(curr));
-                       add_chain_block(curr + req, size - req);
+                       if (size > req)
+                               add_chain_block(curr + req, size - req);
                        return curr;
                }
        }