Re: [PATCH 1/2] PCI/MSI: Cache the MSIX table size

[Leon Romanovsky]

On Tue, Jan 24, 2023 at 02:42:11PM +0200, Alexander Shishkin wrote:
> Leon Romanovsky <leon@xxxxxxxxxx> writes:
> 
> > On Tue, Jan 24, 2023 at 01:52:37PM +0200, Alexander Shishkin wrote:
> >> Leon Romanovsky <leon@xxxxxxxxxx> writes:
> >> 
> >> > I'm not security expert here, but not sure that this protects from anything.
> >> > 1. Kernel relies on working and not-malicious HW. There are gazillion ways
> >> > to cause crashes other than changing MSI-X.
> >> 
> >> This particular bug was preventing our fuzzing from going deeper into
> >> the code and reaching some more of the aforementioned gazillion bugs.
> >
> > Your commit message says nothing about fuzzing, but talks about
> > malicious device. 
> 
> A malicious device is what the fuzzing is aiming to simulate. The fact
> of fuzzing process itself didn't seem relevant to the patch, so I didn't
> include it, going instead for the problem statement and proposed
> solution. Will the commit message benefit from mentioning fuzzing?

No, for most if not all kernel developers, the fuzzing means some sort of
random user-space input. PCI devices are trusted in the kernel.

> 
> > Do you see "gazillion bugs" for devices which don't change their MSI-X
> > table size under the hood, which is main kernel assumption?
> 
> Not so far.

So please share them with us.

> 
> > If yes, you should fix these bugs.
> 
> That's absolutely the intention.

So let's fix the bugs and not hide them.

> 
> >> > 2. Device can report large table size, kernel will cache it and
> >> > malicious device will reduce it back. It is not handled and will cause
> >> > to kernel crash too.
> >> 
> >> How would that happen? If the device decides to have fewer vectors,
> >> they'll all still fit in the ioremapped MSIX table. The worst thing that
> >> can happen is 0xffffffff reads from the mmio space, which a device can
> >> do anyway. But that shouldn't trigger a page fault or otherwise
> >> crash. Or am I missing something?
> >
> > Like I said, I'm no expert. You should tell me if it safe for all
> > callers of pci_msix_vec_count().
> 
> Well, since you stated that the reverse will cause a kernel crash, I had
> to ask how. I'll include some version of the above paragraph in the
> commit message to indicate that we reverse situation has been considered.

Not really. I didn't see any explanation how will it work if number
of vectors (which MSI-X table represents) is completely different from
seeing by PCI core.

Thanks

> 
> Regards,
> --
> Alex