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SubjectRe: [RFC PATCH 1/2] binfmt_elf: Fix corner case kfree of uninitialized data
On Wed, 19 Sep 2012 15:44:35 +0100
Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:

> Could do with double checking...
>
> From: Alan Cox <alan@linux.intel.com>
>
> If elf_core_dump is called and fill_note_info fails in the kmalloc then
> it returns 0 but has not yet initialised all the needed fields. As a result
> we do a kfree(randomness) after correctly skipping the thread data.
>
> Signed-off-by: Alan Cox <alan@linux.intel.com>
> ---
>
> fs/binfmt_elf.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 1b4efbc..bf6d82b 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -1492,8 +1492,10 @@ static int fill_note_info(struct elfhdr *elf, int phdrs,
> info->thread = NULL;
>
> psinfo = kmalloc(sizeof(*psinfo), GFP_KERNEL);
> - if (psinfo == NULL)
> + if (psinfo == NULL) {
> + info->psinfo.data = NULL; /* So we don't free this wrongly */
> return 0;
> + }
>
> fill_note(&info->psinfo, "CORE", NT_PRPSINFO, sizeof(*psinfo), psinfo);

afaict it's NotABug, because fill_note_info() does

info->thread = NULL;

psinfo = kmalloc(sizeof(*psinfo), GFP_KERNEL);
if (psinfo == NULL) {

and free_note_info() does

struct elf_thread_core_info *threads = info->thread;
while (threads) {

so free_note_info() won't enter the freeing loop at all.

Which is just as well, because info->thread_notes is uninitialised at
this time.


It all looks rather fragile - I'm wondering if it would be sanest to
memset `info' right at the outset in elf_core_dump(), then weed out all
the now-unneeded zeroizings in fill_note_info().


Also, how irritating is it that fill_note_info() has a local var
`psinfo' which has a different type from info->psinfo. That had me
running around for a while...


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