lib/buildid: take into account e_phoff when fetching program headers

Current code assumption is that program (segment) headers are following
ELF header immediately. This is a common case, but is not guaranteed. So
take into account e_phoff field of the ELF header when accessing program
headers.

Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240829174232.3133883-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: anakryiko

lib/buildid.c

@@ -213,28 +213,26 @@ static int get_build_id_32(struct freader *r, unsigned char *build_id, __u32 *si
 {
 	const Elf32_Ehdr *ehdr;
 	const Elf32_Phdr *phdr;
-	__u32 phnum, i;
+	__u32 phnum, phoff, i;
 
 	ehdr = freader_fetch(r, 0, sizeof(Elf32_Ehdr));
 	if (!ehdr)
 		return r->err;
 
-	/*
-	 * FIXME
-	 * Neither ELF spec nor ELF loader require that program headers
-	 * start immediately after ELF header.
-	 */
-	if (ehdr->e_phoff != sizeof(Elf32_Ehdr))
-		return -EINVAL;
-
 	/* subsequent freader_fetch() calls invalidate pointers, so remember locally */
 	phnum = READ_ONCE(ehdr->e_phnum);
+	phoff = READ_ONCE(ehdr->e_phoff);
+
 	/* only supports phdr that fits in one page */
 	if (phnum > (PAGE_SIZE - sizeof(Elf32_Ehdr)) / sizeof(Elf32_Phdr))
 		return -EINVAL;
 
+	/* check that phoff is not large enough to cause an overflow */
+	if (phoff + phnum * sizeof(Elf32_Phdr) < phoff)
+		return -EINVAL;
+
 	for (i = 0; i < phnum; ++i) {
-		phdr = freader_fetch(r, i * sizeof(Elf32_Phdr), sizeof(Elf32_Phdr));
+		phdr = freader_fetch(r, phoff + i * sizeof(Elf32_Phdr), sizeof(Elf32_Phdr));
 		if (!phdr)
 			return r->err;
 
@@ -252,27 +250,26 @@ static int get_build_id_64(struct freader *r, unsigned char *build_id, __u32 *si
 	const Elf64_Ehdr *ehdr;
 	const Elf64_Phdr *phdr;
 	__u32 phnum, i;
+	__u64 phoff;
 
 	ehdr = freader_fetch(r, 0, sizeof(Elf64_Ehdr));
 	if (!ehdr)
 		return r->err;
 
-	/*
-	 * FIXME
-	 * Neither ELF spec nor ELF loader require that program headers
-	 * start immediately after ELF header.
-	 */
-	if (ehdr->e_phoff != sizeof(Elf64_Ehdr))
-		return -EINVAL;
-
 	/* subsequent freader_fetch() calls invalidate pointers, so remember locally */
 	phnum = READ_ONCE(ehdr->e_phnum);
+	phoff = READ_ONCE(ehdr->e_phoff);
+
 	/* only supports phdr that fits in one page */
 	if (phnum > (PAGE_SIZE - sizeof(Elf64_Ehdr)) / sizeof(Elf64_Phdr))
 		return -EINVAL;
 
+	/* check that phoff is not large enough to cause an overflow */
+	if (phoff + phnum * sizeof(Elf64_Phdr) < phoff)
+		return -EINVAL;
+
 	for (i = 0; i < phnum; ++i) {
-		phdr = freader_fetch(r, i * sizeof(Elf64_Phdr), sizeof(Elf64_Phdr));
+		phdr = freader_fetch(r, phoff + i * sizeof(Elf64_Phdr), sizeof(Elf64_Phdr));
 		if (!phdr)
 			return r->err;