[PATCH v1] tracing/remotes: Fix page_va[] access before counter update in trace_remote_alloc_buffer()

[Fuad Tabba <fuad.tabba@linux.dev>]

page_va[] is annotated __counted_by(nr_page_va), so nr_page_va must
cover an index before that element is accessed. The allocation loop
writes page_va[id] while nr_page_va is still id and increments it only
afterwards, so every write is one element past the declared count.

The store is out of bounds with respect to the annotation: a build with
CONFIG_UBSAN_BOUNDS on a toolchain that honours __counted_by
(clang >= 20.1, gcc >= 15.1) flags it as an array-index overflow.

Increment nr_page_va before writing the element it now covers. A failed
allocation then leaves the slot counted but NULL; the error path frees
it with free_page(0), which is a no-op.

Fixes: 96e43537af546 ("tracing: Introduce trace remotes")
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
I found this while reviewing Vincent's series [1].

Applies on top of Vincent's "[PATCH v2 0/3] tracing/remotes: Fix leak in
trace_remote_alloc_buffer() error path" [2], which fixes the other
issues in this function.

[1] https://lore.kernel.org/all/20260710114819.2689386-1-vdonnefort@google.com/
[2] https://lore.kernel.org/all/20260709160017.1729517-1-vdonnefort@google.com/

 kernel/trace/trace_remote.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
index 0f6ef5c36d84e..ef42d9c38b374 100644
--- a/kernel/trace/trace_remote.c
+++ b/kernel/trace/trace_remote.c
@@ -1004,11 +1004,10 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
 		desc->nr_cpus++;
 
 		for (id = 0; id < nr_pages; id++) {
+			rb_desc->nr_page_va++;
 			rb_desc->page_va[id] = (unsigned long)__get_free_page(GFP_KERNEL);
 			if (!rb_desc->page_va[id])
 				goto err;
-
-			rb_desc->nr_page_va++;
 		}
 		rb_desc = __next_ring_buffer_desc(rb_desc);
 	}

base-commit: 8cdeaa50eae8dad34885515f62559ee83e7e8dda
prerequisite-patch-id: 8bdd7b28f09e97cff4f5995196b83b377c961d8b
prerequisite-patch-id: 6781b7ce5156d229fd21a757ef0cd66dd76130cc
prerequisite-patch-id: 56bcbbdceddf4533770d48d1835d5b7e11f47d56
-- 
2.39.5