[PATCH v2] binfmt_elf: Allow .bss in any interp PT_LOAD

Kees Cook
2022-11-11T06:13:28Z

Traditionally, only the final PT_LOAD for load_elf_interp() supported
having p_memsz > p_filesz. Recently, lld's construction of musl's
libc.so on PowerPC64 started having two PT_LOAD program headers with
p_memsz > p_filesz.

As the least invasive change possible, check for p_memsz > p_filesz for
each PT_LOAD in load_elf_interp.

Reported-by: Rich Felker <dalias@libc.org>
Link: https://maskray.me/blog/2022-11-05-lld-musl-powerpc64
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Fangrui Song <maskray@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
v2: I realized we need to retain the final padding call.
v1: https://lore.kernel.org/linux-hardening/20221111055747.never.202-kees@kernel.org/
---
fs/binfmt_elf.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 528e2ac8931f..0a24bbbef1d6 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -673,15 +673,25 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
last_bss = k;
bss_prot = elf_prot;
}
+
+ /*
+ * Clear any p_memsz > p_filesz area up to the end
+ * of the page to wipe anything left over from the
+ * loaded file contents.
+ */
+ if (last_bss > elf_bss && padzero(elf_bss))
+ error = -EFAULT;
+ goto out;
+ }
}
}

/*
- * Now fill out the bss section: first pad the last page from
- * the file up to the page boundary, and zero it from elf_bss
- * up to the end of the page.
+ * Finally, pad the last page from the file up to the page boundary,
+ * and zero it from elf_bss up to the end of the page, if this did
+ * not already happen with the last PT_LOAD.
*/
- if (padzero(elf_bss)) {
+ if (last_bss == elf_bss && padzero(elf_bss)) {
error = -EFAULT;
goto out;
}
--
2.34.1