[PING][PATCH] binfmt_elf: Don't clobber passed executable's file header

From: Maciej W. Rozycki
Date: Mon Oct 26 2015 - 11:48:28 EST

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>

Is there any technical issue with this change? I'll be happy to address
any. Otherwise any chance to have it included in 4.4?


Index: linux-org-test/fs/binfmt_elf.c
--- linux-org-test.orig/fs/binfmt_elf.c 2015-07-18 00:11:13.000000000 +0100
+++ linux-org-test/fs/binfmt_elf.c 2015-07-18 00:32:41.509841000 +0100
@@ -759,16 +759,16 @@ static int load_elf_binary(struct linux_
would_dump(bprm, interpreter);

- retval = kernel_read(interpreter, 0, bprm->buf,
- if (retval != BINPRM_BUF_SIZE) {
+ /* Get the exec headers */
+ retval = kernel_read(interpreter, 0,
+ &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);

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