Re: [PATCH] pstore/ram: fix for adding dumps to non-empty zone

[Kees Cook]

On Tue, Dec 31, 2019 at 10:00:34AM +0200, Nikolai Merinov wrote:
> On Dec 31, 2019, at 1:37 AM, Kees Cook keescook@xxxxxxxxxxxx wrote:
> > On Mon, Dec 23, 2019 at 06:38:16PM +0500, Nikolai Merinov wrote:
> >> From: Aleksandr Yashkin <a.yashkin@xxxxxxxxxxxxxxxxxx>
> >> 
> >> The circle buffer in ramoops zones has a problem for adding a new
> >> oops dump to already an existing one.
> >> 
> >> The solution to this problem is to reset the circle buffer state before
> >> writing a new oops dump.
> > 
> > Ah, I see it now. When the crashes wrap around, the header is written at
> > the end of the (possibly incompletely filled) buffer, instead of at the
> > start, since it wasn't explicitly zapped.
> 
> Yes, you are right. We observed this issue when we got two consecutive
> reboots with a kernel panic: After the first reboot the pstore was able
> to parse the saved data, but after the second we got a part of the
> previous compressed message and the new compressed message with the
> ramoops message header at the middle of the file. 
> 
> According to our analysis the same issue can occur if we get more
> than (rampoops.mem_size/ramoops.record_size) kernel oops during work.

Yup, perfect. Thanks for confirming! I've tested the patches with this
in mind and everything seems to be working correctly.

> 
> > 
> > Yes, this is important; thank you for tracking this down! This bug has
> > existed for a very long time. I'll try to find the right Fixes tag for
> > it...
> 
> We would like to see this changes in the LTS kernel releases. Could you
> please point me the manual about propagation such fixes?

You don't need to do anything. :) I have already marked it for -stable,
and it'll end up there as it makes its way through the development
process.

-Kees

> 
> > 
> > -Kees
> > 
> >> Signed-off-by: Aleksandr Yashkin <a.yashkin@xxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: Nikolay Merinov <n.merinov@xxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: Ariel Gilman <a.gilman@xxxxxxxxxxxxxxxxxx>
> >> 
> >> diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
> >> index 8caff834f002..33fceadbf515 100644
> >> --- a/fs/pstore/ram.c
> >> +++ b/fs/pstore/ram.c
> >> @@ -407,6 +407,13 @@ static int notrace ramoops_pstore_write(struct
> >> pstore_record *record)
> >>  
> >>  	prz = cxt->dprzs[cxt->dump_write_cnt];
> >>  
> >> +	/* Clean the buffer from old info.
> >> +	 * `ramoops_read_kmsg_hdr' expects to find a header in the beginning of
> >> +	 * buffer data, so we must to reset the buffer values, in order to
> >> +	 * ensure that the header will be written to the beginning of the buffer
> >> +	 */
> >> +	persistent_ram_zap(prz);
> >> +
> >>  	/* Build header and append record contents. */
> >>  	hlen = ramoops_write_kmsg_hdr(prz, record);
> >>  	if (!hlen)
> >> --
> >> 2.17.1
> >> 
> > 
> > --
> > Kees Cook
> --
> Nikolai Merinov

-- 
Kees Cook