Re: [PATCH 4.9] bpf: fix overflow in prog accounting

[Greg KH <greg@kroah.com>]

On Fri, Aug 12, 2022 at 10:22:11AM +0100, Quentin Monnet wrote:
> From: Daniel Borkmann <daniel@iogearbox.net>
> 
> [ Upstream commit 5ccb071e97fbd9ffe623a0d3977cc6d013bee93c ]
> 
> Commit aaac3ba95e4c ("bpf: charge user for creation of BPF maps and
> programs") made a wrong assumption of charging against prog->pages.
> Unlike map->pages, prog->pages are still subject to change when we
> need to expand the program through bpf_prog_realloc().
> 
> This can for example happen during verification stage when we need to
> expand and rewrite parts of the program. Should the required space
> cross a page boundary, then prog->pages is not the same anymore as
> its original value that we used to bpf_prog_charge_memlock() on. Thus,
> we'll hit a wrap-around during bpf_prog_uncharge_memlock() when prog
> is freed eventually. I noticed this that despite having unlimited
> memlock, programs suddenly refused to load with EPERM error due to
> insufficient memlock.
> 
> There are two ways to fix this issue. One would be to add a cached
> variable to struct bpf_prog that takes a snapshot of prog->pages at the
> time of charging. The other approach is to also account for resizes. I
> chose to go with the latter for a couple of reasons: i) We want accounting
> rather to be more accurate instead of further fooling limits, ii) adding
> yet another page counter on struct bpf_prog would also be a waste just
> for this purpose. We also do want to charge as early as possible to
> avoid going into the verifier just to find out later on that we crossed
> limits. The only place that needs to be fixed is bpf_prog_realloc(),
> since only here we expand the program, so we try to account for the
> needed delta and should we fail, call-sites check for outcome anyway.
> On cBPF to eBPF migrations, we don't grab a reference to the user as
> they are charged differently. With that in place, my test case worked
> fine.
> 
> Fixes: aaac3ba95e4c ("bpf: charge user for creation of BPF maps and programs")
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> Acked-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: David S. Miller <davem@davemloft.net>
> [Quentin: backport to 4.9: Adjust context in bpf.h ]
> Signed-off-by: Quentin Monnet <quentin@isovalent.com>
> ---
> This fix was merged in Linux 4.10 but never backported to 4.9. The
> overflow has been occurring regularly when running Cilium's CI tests on
> kernel 4.9, so I would like to submit this patch for consideration to
> the 4.9 stable branch.
> 
> The initial patch applied with a minor conflict on include/linux/bpf.h,
> due to unprivileged_ebpf_enabled() backported in 6481835a9a5b
> ("x86/speculation: Include unprivileged eBPF status in Spectre v2
> mitigation reporting")

Now queued up, thanks.

greg k-h