From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	sashiko-bot <sashiko-bot@kernel.org>,
	Ian Rogers <irogers@google.com>,
	Arnaldo Carvalho de Melo <acme@redhat.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.12 1142/1376] perf dso: Guard against cache underflow on short reads in dso_cache__memcpy()
Date: Sat, 12 Sep 2026 08:59:28 +0200
Message-ID: <20260912065633.009436111@linuxfoundation.org>
In-Reply-To: <20260912065607.535295758@linuxfoundation.org>
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6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Arnaldo Carvalho de Melo <acme@redhat.com>

[ Upstream commit 390a9461cd73bdd13acc0f6d763618ae1ff8fa17 ]

dso_cache__memcpy() computes cache_offset = offset - cache->offset,
then cache_size = min(cache->size - cache_offset, size).  The RB tree
lookup in __dso_cache__find() matches using the full
DSO__DATA_CACHE_SIZE window, but cache->size reflects the actual pread
return value from dso_cache__populate().

A short pread (e.g. near end-of-file) makes cache->size smaller than
DSO__DATA_CACHE_SIZE.  If a subsequent access targets an offset past
cache->offset + cache->size but within the DSO__DATA_CACHE_SIZE
window, the cache entry is found but cache_offset exceeds cache->size.
Since both are u64, the subtraction cache->size - cache_offset wraps
to a large value, min() selects the caller's size, and memcpy reads
out of bounds.

Return 0 for an offset past the valid cached data.  For a regular
file a short pread only happens at end-of-file, so 0 is what a direct
pread() at that offset would return: cached_io() stops its read loop
as on EOF.  Re-reading from the backing file would not help — a
second pread at the same offset returns the same short count.

Fixes: 366df72657e0 ("perf dso: Refactor dso_cache__read()")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Reviewed-by: Ian Rogers <irogers@google.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 tools/perf/util/dso.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
index 266f3b0a3a54b..9de78061a3f26 100644
--- a/tools/perf/util/dso.c
+++ b/tools/perf/util/dso.c
@@ -939,7 +939,20 @@ static ssize_t dso_cache__memcpy(struct dso_cache *cache, u64 offset, u8 *data,
 				 u64 size, bool out)
 {
 	u64 cache_offset = offset - cache->offset;
-	u64 cache_size   = min(cache->size - cache_offset, size);
+	u64 cache_size;
+
+	/*
+	 * The RB tree matches using DSO__DATA_CACHE_SIZE, but a short
+	 * pread may leave cache->size smaller.  For a regular file a
+	 * short pread only happens at end-of-file, so an offset past
+	 * the valid data is EOF: return 0, matching what a direct
+	 * pread() at that offset would return, and cached_io() then
+	 * stops its read loop.
+	 */
+	if (cache_offset >= cache->size)
+		return 0;
+
+	cache_size = min(cache->size - cache_offset, size);
 
 	if (out)
 		memcpy(data, cache->data + cache_offset, cache_size);
-- 
2.53.0