From 0007654a7bf13ef2a19afd398bbc6507087f19fd Mon Sep 17 00:00:00 2001 From: Nickolai Zeldovich Date: Sat, 17 Oct 2026 20:07:57 -0400 Subject: [PATCH] lib/buildid: reject file offsets that do not fit in pgoff_t On a 32-bit kernel, parsing the build ID of a crafted ELF file can copy far more than the intended bytes into a 64-byte on-stack buffer in __build_id_parse(), corrupting the stack of whatever context fetches the build ID. This might be reachable without privilege: perf records a build ID for PERF_RECORD_MMAP2 on the no-fault path when a process mmaps its own file (build_id_parse_nofault() from perf_event_mmap()), and BPF build-id stack maps do the same. freader_get_folio() turns a 64-bit file offset into a page cache index with file_off >> PAGE_SHIFT and passes it to filemap_get_folio(). pgoff_t is unsigned long, 32-bit on a 32-bit kernel, so an ELF64 e_phoff of 2^44 + k * PAGE_SIZE truncates to index k. filemap_get_folio() returns the cached folio at index k, whose folio_pos() is far below the requested file_off. In freader_fetch() the folio does not contain file_off, so the "crossing folio boundaries" path computes part_sz = folio_off + folio_sz - file_off as a large u64 (file_off is much larger), and the following memcpy() truncates it to size_t and copies a page into the 64-byte buffer. On a 64-bit kernel the index is not truncated, the folio is not cached and the fetch returns -EFAULT, so only 32-bit kernels are affected. Reject a file offset that cannot be represented as a page cache index before the lookup, so such a file fails the build ID parse cleanly as it already does on 64-bit. Reproduced on i386 under QEMU with CONFIG_STACKPROTECTOR_STRONG: mmapping a crafted ELF (ELFCLASS64, e_phoff = 2^44) while a perf event with attr.build_id is open panics with "stack-protector: Kernel stack is corrupted in: __build_id_parse"; with this patch the parse returns -EFAULT and the mmap completes cleanly. Built for i386 with W=1, no new warnings. Fixes: de3ec364c3c3 ("lib/buildid: add single folio-based file reader abstraction") Assisted-by: LLM Cc: stable@vger.kernel.org Signed-off-by: Nickolai Zeldovich Signed-off-by: Andrii Nakryiko --- lib/buildid.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/lib/buildid.c b/lib/buildid.c index c4b737640621..995a327f46ae 100644 --- a/lib/buildid.c +++ b/lib/buildid.c @@ -47,6 +47,10 @@ static int freader_get_folio(struct freader *r, loff_t file_off) freader_put_folio(r); + /* the page index must fit in pgoff_t */ + if (file_off < 0 || file_off > MAX_LFS_FILESIZE) + return -EFAULT; + /* only use page cache lookup - fail if not already cached */ r->folio = filemap_get_folio(r->file->f_mapping, file_off >> PAGE_SHIFT);