[PATCH 12/23] perf symbols: Bounds-check descsz in sysfs__read_build_id() GNU fallback

[Arnaldo Carvalho de Melo]

From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>

When sysfs__read_build_id() matches NT_GNU_BUILD_ID with the right
namesz but the name content is not "GNU", it falls back to reading
descsz bytes into the stack buffer bf[BUFSIZ]:

    } else if (read(fd, bf, descsz) != (ssize_t)descsz)

Unlike the else branch which validates namesz + descsz against
sizeof(bf), this path passes descsz directly to read() without any
bounds check.  A crafted sysfs file with a large n_descsz overflows
the 8192-byte stack buffer.

Add a descsz > sizeof(bf) check before the read, breaking out of
the loop on oversized values.

Fixes: e5a1845fc0aeca85 ("perf symbols: Split out util/symbol-elf.c")
Reported-by: sashiko-bot <sashiko-bot@xxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Assisted-by: Claude Opus 4.6 <noreply@xxxxxxxxxxxxx>
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
---
 tools/perf/util/symbol-elf.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 36a0304707e13138..06cfb84f86eb2f64 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -961,8 +961,13 @@ int sysfs__read_build_id(const char *filename, struct build_id *bid)
 					err = 0;
 					break;
 				}
-			} else if (read(fd, bf, descsz) != (ssize_t)descsz)
-				break;
+			} else {
+				/* descsz from untrusted file — clamp to buffer */
+				if (descsz > sizeof(bf))
+					break;
+				if (read(fd, bf, descsz) != (ssize_t)descsz)
+					break;
+			}
 		} else {
 			size_t n;
 
-- 
2.54.0