[PATCH 3.16.y-ckt 084/126] binfmt_elf: Don't clobber passed executable's file header

[Luis Henriques]

3.16.7-ckt21 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Maciej W. Rozycki" <macro@xxxxxxxxxx>

commit b582ef5c53040c5feef4c96a8f9585b6831e2441 upstream.

Do not clobber the buffer space passed from `search_binary_handler' and
originally preloaded by `prepare_binprm' with the executable's file
header by overwriting it with its interpreter's file header.  Instead
keep the buffer space intact and directly use the data structure locally
allocated for the interpreter's file header, fixing a bug introduced in
2.1.14 with loadable module support (linux-mips.org commit beb11695
[Import of Linux/MIPS 2.1.14], predating kernel.org repo's history).
Adjust the amount of data read from the interpreter's file accordingly.

This was not an issue before loadable module support, because back then
`load_elf_binary' was executed only once for a given ELF executable,
whether the function succeeded or failed.

With loadable module support supported and enabled, upon a failure of
`load_elf_binary' -- which may for example be caused by architecture
code rejecting an executable due to a missing hardware feature requested
in the file header -- a module load is attempted and then the function
reexecuted by `search_binary_handler'.  With the executable's file
header replaced with its interpreter's file header the executable can
then be erroneously accepted in this subsequent attempt.

Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 fs/binfmt_elf.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index c13a138c1016..57464b34d153 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -683,16 +683,16 @@ static int load_elf_binary(struct linux_binprm *bprm)
 			 */
 			would_dump(bprm, interpreter);
 
-			retval = kernel_read(interpreter, 0, bprm->buf,
-					     BINPRM_BUF_SIZE);
-			if (retval != BINPRM_BUF_SIZE) {
+			/* Get the exec headers */
+			retval = kernel_read(interpreter, 0,
+					     (void *)&loc->interp_elf_ex,
+					     sizeof(loc->interp_elf_ex));
+			if (retval != sizeof(loc->interp_elf_ex)) {
 				if (retval >= 0)
 					retval = -EIO;
 				goto out_free_dentry;
 			}
 
-			/* Get the exec headers */
-			loc->interp_elf_ex = *((struct elfhdr *)bprm->buf);
 			break;
 		}
 		elf_ppnt++;
--
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