perfmon trouble

[Al Viro]

On Sat, Jun 09, 2018 at 04:51:08PM +0100, Al Viro wrote:

> Stephane, could you comment on the situation in there?  I realize that you
> hadn't touched that thing in more than a decade, but I've no idea who else
> might be familiar with that thing and it's very inconveniently special...

Having looked through that code... ouch.  It tries to have munmap-on-close,
of all things.  Which has interesting consequences; consider, for example,
	fd = perfctl(-1, PFM_CREATE_CONTEXT, &blah, 1);	// create a context
	....
	pid = fork();
	if (!pid) {
		execve("/usr/bin/something_suid", ...);
		...
	}

with something_suid(8) doing an explicit "close each descriptor past stdout"
loop.

PFM_CREATE_CONTEXT has created a context, mmapped its buffer (and stored
the address of that mapping in ctx->ctx_smpl_vaddr) and, having opened
an associated file, sticks it into descriptor table and returns the descriptor.

On fork/exec we have
	* descriptor table copied to child
	* all mappings copied to child and then destroyed by execve
	* execve ends up with the new binary (and libraries, etc.) mmapped
(in child)

Now, our careful suid-root binary does close(2) on its copy of descriptor.
pfm_flush() is called.  ctx->task != current, so we proceed to
        /*
         * remove virtual mapping, if any, for the calling task.
         * cannot reset ctx field until last user is calling close().
         *
         * ctx_smpl_vaddr must never be cleared because it is needed
         * by every task with access to the context
         *
         * When called from do_exit(), the mm context is gone already, therefore
         * mm is NULL, i.e., the VMA is already gone  and we do not have to
         * do anything here
         */
        if (ctx->ctx_smpl_vaddr && current->mm) {
                smpl_buf_vaddr = ctx->ctx_smpl_vaddr;
                smpl_buf_size  = ctx->ctx_smpl_size;
        }

        UNPROTECT_CTX(ctx, flags);

        /*
         * if there was a mapping, then we systematically remove it
         * at this point. Cannot be done inside critical section
         * because some VM function reenables interrupts.
         *
         */
        if (smpl_buf_vaddr) pfm_remove_smpl_mapping(smpl_buf_vaddr, smpl_buf_size);

... with the last call doing vm_munmap() on the area in question.  In the
address space of that suid-root binary, taking out whatever *it* had mapped
at that address range...

I wouldn't be surprised if that turned out to be realistically exploitable ;-/
Is there any documentation of that thing's semantics?  perfmonctl(2) doesn't
mention the mapping at all and link to HP site in the arch/ia64/kernel/perfmon.c
is 404-compliant.  Playing with archive.org brings a sourceforget reference,
but I hadn't been able to find anything ia64-related docs in there...