[PATCH] binfmt_flat: make load_flat_shared_library() work

[Jann Horn <jannh@google.com>]

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@vger.kernel.org
Fixes: 287980e49ffc ("remove lots of IS_ERR_VALUE abuses")
Signed-off-by: Jann Horn <jannh@google.com>
---
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);
-
-out:
 	allow_write_access(bprm.file);
 	fput(bprm.file);
 
-- 
2.22.0.rc1.257.g3120a18244-goog