Re: [PATCH v8 3/3] ring-buffer: Prevent truncation of nr_pages / nr_subbufs
From: sashiko-bot@kernel.org
Date: 2026-08-26 10:02:18
Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider: Pre-existing issues: - [High] The kernel fails to validate the untrusted `meta->nr_subbufs` value from persistent memory against the expected `nr_pages + 1`, leading to arbitrary heap out-of-bounds writes and potential arbitrary kernel memory corruption. -- commit 5ed0ab549782b2ff87e1bd2394abb78b10c4b602 Author: Vincent Donnefort [off-list ref] ring-buffer: Prevent truncation of nr_pages / nr_subbufs Although ring_buffer_per_cpu::nr_pages is defined as unsigned long, it is capped to 32-bits in a few places, limiting the operations possible on a very large buffer. Use unsigned long where appropriate and prevent truncation of values using nr_pages (or nr_subbufs). While at it, subbuf_size must be at least unsigned int. Note that persistent, remote and user-mapped ring buffers are capping the number of pages to 31 bits already, making "int" safe in many places.
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diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 6089fcc67e2b6..1fcf55e2ca81f 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c@@ -1745,7 +1744,7 @@ static void *rb_range_meta(struct trace_buffer *buffer, int nr_pages, int cpu) /* Return the start of subbufs given the meta pointer */ static void *rb_subbufs_from_meta(struct ring_buffer_cpu_meta *meta) { - int subbuf_size = meta->subbuf_size; + unsigned int subbuf_size = meta->subbuf_size; unsigned long ptr; ptr = (unsigned long)meta;
[Severity: High] This isn't a bug introduced by this patch, but does the next line in this function perform pointer arithmetic using an unvalidated meta->nr_subbufs? ptr = rb_range_align_subbuf(ptr, subbuf_size, meta->nr_subbufs); Could this yield a corrupted or attacker-controlled subbuf base pointer if meta->nr_subbufs is read from persistent memory without being checked against expected limits? [ ... ]
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@@ -1854,13 +1853,13 @@ static bool rb_meta_init(struct trace_buffer *buffer, int scratch_size) * must be the same. */ static bool rb_cpu_meta_valid(struct ring_buffer_cpu_meta *meta, int cpu, - struct trace_buffer *buffer, int nr_pages, + struct trace_buffer *buffer, unsigned long nr_pages, unsigned long *subbuf_mask) { - int subbuf_size = PAGE_SIZE; + unsigned long subbuf_size = PAGE_SIZE; unsigned long buffers_start; unsigned long buffers_end; - int i; + unsigned long i; if (!subbuf_mask) return false;
[Severity: High]
This isn't a bug introduced by this patch, but is there a risk of an
out-of-bounds heap write further down in this function?
The subbuf_mask is allocated with nr_pages + 1 bits in rb_range_meta_init(),
but later in rb_cpu_meta_valid() operations use the unvalidated
meta->nr_subbufs value read from persistent memory:
bitmap_clear(subbuf_mask, 0, meta->nr_subbufs);
/*
* Ensure the meta::buffers array has correct data. The data in each subbufs
* are checked later in rb_meta_validate_events().
*/
for (i = 0; i < meta->nr_subbufs; i++) {
if (meta->buffers[i] < 0 ||
meta->buffers[i] >= meta->nr_subbufs) {
pr_info("Ring buffer boot meta [%d] array out of range\n", cpu);
return false;
}
if (test_bit(meta->buffers[i], subbuf_mask)) {
pr_info("Ring buffer boot meta [%d] array has duplicates\n", cpu);
return false;
}
set_bit(meta->buffers[i], subbuf_mask);
}
If meta->nr_subbufs is larger than nr_pages + 1, could this write out of
bounds and corrupt kernel memory?
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Sashiko AI review · https://sashiko.dev/#/patchset/20260826094528.3738023-1-vdonnefort@google.com?part=3