Re: [PATCH] i2c: i801: Fix memory corruption in i801_isr_byte_done()

[Jean Delvare <jdelvare@suse.de>]

On Mon, 23 Mar 2020 12:37:33 +0300, Dan Carpenter wrote:
> On Sun, Mar 22, 2020 at 11:11:06PM +0100, Jean Delvare wrote:
> > Definitely not correct. The first byte of the block data array MUST be
> > the size of the block read. Even if the code above does not do the
> > right thing, removing the line will not help.
> >   
> 
> Yeah.  I misread the code.
> 
> > Is it possible that kasan got this wrong due to the convoluted logic?
> > It's late and I'll check again tomorrow morning but the code looks OK
> > to me.  
> 
> KASan doesn't work like that.  It works at runtime and doesn't care
> about the logic.
> 
> https://syzkaller.appspot.com/bug?id=426fc8b1c1b63fb0af524d839dfcf452f2d858e2
> 
> At the bottom of the report it shows that we're in a field of f9
> poisoned data so it's not priv->len which is wrong.  (My patch was way
> off).
> 
> mm/kasan/kasan.h:#define KASAN_VMALLOC_INVALID   0xF9  /* unallocated space in vmapped page */
> 
> The logic looks okay to me too.  So possibly this was a race condition
> or even memory corruption in an unrelated part of the kernel.

I checked out the exact kernel version this report was generated for,
and the faulty line is:

  592:			priv->data[priv->count++] = inb(SMBBLKDAT(priv));

This would suggest the problem is with priv->count growing beyond the
end of the array, however the fact that we land in a memory spot full
of 0xF9 kind of excludes this possibility (the data before the spot
would contain different data if it was the case).

The other option is that priv->count wasn't initialized at the time
it is used. However I can't see how this could happen, given that the
priv structure is kzalloc'd.

So, to be honest I can't really see how priv->count can get wrong. So
I would be tempted to lend towards the theory that the i2c-i801 driver
was a collateral victim of a memory corruption happening somewhere else
in the kernel. Wouldn't Kasan catch this too? Is it possible to access
the other Kasan reports from the same test run?

-- 
Jean Delvare
SUSE L3 Support