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SubjectRe: [PATCH, RESEND] ipc/shm: handle removed segments gracefully in shm_mmap()
On Wed, 11 Nov 2015, Kirill A. Shutemov wrote:

>remap_file_pages(2) emulation can reach file which represents removed
>IPC ID as long as a memory segment is mapped. It breaks expectations
>of IPC subsystem.
>
>Test case (rewritten to be more human readable, originally autogenerated
>by syzkaller[1]):
>
> #define _GNU_SOURCE
> #include <stdlib.h>
> #include <sys/ipc.h>
> #include <sys/mman.h>
> #include <sys/shm.h>
>
> #define PAGE_SIZE 4096
>
> int main()
> {
> int id;
> void *p;
>
> id = shmget(IPC_PRIVATE, 3 * PAGE_SIZE, 0);
> p = shmat(id, NULL, 0);
> shmctl(id, IPC_RMID, NULL);
> remap_file_pages(p, 3 * PAGE_SIZE, 0, 7, 0);
>
> return 0;
> }
>
>The patch changes shm_mmap() and code around shm_lock() to propagate
>locking error back to caller of shm_mmap().
>
>[1] http://github.com/google/syzkaller

So this is a very similar approach that I posted back when this discussion
arose: https://lkml.org/lkml/2015/10/12/959 -- There are a few differences
for which I prefer mine :)

o My shm_check_vma_validity() also deals with IPC_RMID as we do the
ipc_valid_object() check.

o We have a new WARN where necessary, instead of having one now is shm_open.

o My no-ops explicitly pair.

[...]

> ret = sfd->file->f_op->mmap(sfd->file, vma);
>- if (ret != 0)
>+ if (ret) {
>+ shm_close(vma);
> return ret;
>+ }

Hmm what's this shm_close() about?

Thanks,
Davidlohr


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