[PATCH] binfmt_flat: make load_flat_shared_library() work

From: Jann Horn
Date: Fri May 24 2019 - 16:21:43 EST

load_flat_shared_library() is broken: It only calls load_flat_file() if
prepare_binprm() returns zero, but prepare_binprm() returns the number of
bytes read - so this only happens if the file is empty.

Instead, call into load_flat_file() if the number of bytes read is
non-negative. (Even if the number of bytes is zero - in that case,
load_flat_file() will see nullbytes and return a nice -ENOEXEC.)

In addition, remove the code related to bprm creds and stop using
prepare_binprm() - this code is loading a library, not a main executable,
and it only actually uses the members "buf", "file" and "filename" of the
linux_binprm struct. Instead, call kernel_read() directly.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 287980e49ffc ("remove lots of IS_ERR_VALUE abuses")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
I only found the bug by looking at the code, I have not verified its
existence at runtime.
Also, this patch is compile-tested only.
It would be nice if someone who works with nommu Linux could have a
look at this patch.
akpm's tree is the right one for this patch, right?

fs/binfmt_flat.c | 23 +++++++----------------
1 file changed, 7 insertions(+), 16 deletions(-)

diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
index 82a48e830018..e4b59e76afb0 100644
--- a/fs/binfmt_flat.c
+++ b/fs/binfmt_flat.c
@@ -856,9 +856,14 @@ static int load_flat_file(struct linux_binprm *bprm,

static int load_flat_shared_library(int id, struct lib_info *libs)
+ /*
+ * This is a fake bprm struct; only the members "buf", "file" and
+ * "filename" are actually used.
+ */
struct linux_binprm bprm;
int res;
char buf[16];
+ loff_t pos = 0;

memset(&bprm, 0, sizeof(bprm));

@@ -872,25 +877,11 @@ static int load_flat_shared_library(int id, struct lib_info *libs)
if (IS_ERR(bprm.file))
return res;

- bprm.cred = prepare_exec_creds();
- res = -ENOMEM;
- if (!bprm.cred)
- goto out;
- /* We don't really care about recalculating credentials at this point
- * as we're past the point of no return and are dealing with shared
- * libraries.
- */
- bprm.called_set_creds = 1;
+ res = kernel_read(bprm.file, bprm.buf, BINPRM_BUF_SIZE, &pos);

- res = prepare_binprm(&bprm);
- if (!res)
+ if (res >= 0)
res = load_flat_file(&bprm, libs, id, NULL);

- abort_creds(bprm.cred);