[PATCH,v2] arm64: fix the illegal address access in some cases

[Will Deacon]

Hi Robin,

On Tue, Jul 28, 2020 at 03:30:50PM +0100, Robin Murphy wrote:
> On 2020-07-28 14:03, Will Deacon wrote:
> > On Sat, 25 Jul 2020 10:08:06 +0800, guodeqing wrote:
> > > The ihl value of ip header is smaller than 5 in some cases, if the
> > > ihl value is smaller than 5, then the next code will access the illegal
> > > address, and the system will panic. ip_fast_csum() must be able to handle
> > > any value that could fit in the ihl field of the ip protocol header.
> > > 
> > > Here I add the check of the ihl value to solve this problem.
> > 
> > Applied to arm64 (for-next/fixes), thanks!
> > 
> > [1/1] arm64: csum: Reject IP headers with 'ihl' field smaller than five
> >        https://git.kernel.org/arm64/c/09aaef1c5f50
> 
> I'm not sure your commit message is entirely right there. AFAICS it's not
> "the same way as x86" at all - x86 dereferences the first word of iph and
> returns that as the sum if ihl <= 4 (and thus is still capable of crashing
> given sufficiently bogus data). I'm not sure where "return 1" came from - if
> we're going to return nonsense then the mildly more efficient choice of 0
> seems just as good.

Argh, yes, that's %1 not $1, so I don't know where the 1 comes from either.
Geffrey?

> Otherwise it would seem reasonable to jump straight into
> the word-at-a-time loop if ip_fast_csum() is really expected to cope with
> more than just genuine IP headers (which should be backed by at least 20
> bytes of valid memory regardless of what ihl says).

Either copying the x86 behaviour or WARN_ON_ONCE() and assuming and ihl of 5
would be my preference, because I agree with you that this feels like it
shouldn't be happening to start with.

> I still think this smells of papering over some other bug that led to a
> bogus skb getting that far into the transmit stack in the first place -
> presumably it's all wasted effort anyway since a "header" with no space for
> a destination address and a deliberately wrong checksum seems unlikely to go
> very far...

Looking at the ipvlan_start_xmit() path from the backtrace, it looks to
me like ipvlan_get_L3_hdr() returns NULL if the header length is invalid,
but then ipvlan_xmit_mode_l3() ends up calling ipvlan_process_outbound()
anyway. Hmm. I really don't know enough about VLANs to know what the right
behaviour is here and I guess just returning NET_XMIT_DROP will break
something.

> On a quick look there appear to be quite a few implementations dereferencing
> up to 5 words unconditionally, so it's not like this is arm64's own bug.

I'll drop the patch, but we are apparently open to a crash here, so if
you have time to figure out what's going on, that would be great. The
reproducer didn't work for me (I guess I'm missing some utils) and sending
bogus header lengths with a raw socket worked fine (i.e. didn't crash
either). I guess the vlan is an important piece of the picture.

Will