Re: [PATCH] ide-cd: prevent null pointer deref via cdrom_newpc_intr
From: Borislav Petkov
Date: Fri Jun 19 2009 - 04:54:27 EST
Hi,
On Thu, Jun 18, 2009 at 08:25:44PM +0200, Rainer Weikusat wrote:
> Borislav Petkov <petkovbb@xxxxxxxxxxxxxx> writes:
> > On Thu, Jun 18, 2009 at 6:18 PM, Rainer Weikusat<rweikusat@xxxxxxxxxxx> wrote:
> >> Borislav Petkov <petkovbb@xxxxxxxxxxxxxx> writes:
> >>> On Thu, Jun 18, 2009 at 4:52 PM, Rainer Weikusat<rweikusat@xxxxxxxxxxx> wrote:
> >>>> Borislav Petkov <petkovbb@xxxxxxxxxxxxxx> writes:
> >>
> >> [...]
> >>
> >>>>> so this request is completed as a whole and the rq
> >>>>> freed."
> >>>>
> >>>> Technically, this is not quite correct (assuming I haven't overlooked
> >>>> something), because ide_cd_queue_pc still has a reference to the rq.
> >>>
> >>> That doesn't matter because the OOPS happens after the command has been
> >>> issued and _before_ ide_cd_queue_pc() gets to access the rq ptr
> >>> again.
> >>
> >> Yes. Because the pointer I already mentioned has been reset.
>
> And this happens here (!!!, code from 2.6.30 vanilla):
>
> int ide_complete_rq(ide_drive_t *drive, int error, unsigned int nr_bytes)
> {
> ide_hwif_t *hwif = drive->hwif;
> struct request *rq = hwif->rq;
> int rc;
>
> /*
> * if failfast is set on a request, override number of sectors
> * and complete the whole request right now
> */
> if (blk_noretry_request(rq) && error <= 0)
> nr_bytes = rq->hard_nr_sectors << 9;
>
> rc = ide_end_rq(drive, rq, error, nr_bytes);
> if (rc == 0)
> hwif->rq = NULL; /* !!! */
>
> return rc;
> }
> [explanation below]
>
> My first attempt at getting this to work again actually looked like
> this (as addition to cdrom_newpc_intr)
>
> if (uptodate == 0) {
> ide_cd_error_cmd(drive, cmd);
> rq = drive->hwif->rq;
> }
>
> if (rq) {
> /* code up to the 2nd complete call */
> }
>
> if (sense && rc == 2)
> ide_error(drive, "request sense failure", stat);
>
> [...]
>
> That was before I had any idea why complete was being called twice and
> that what this is supposed to do won't be done for bio-less requests,
> anyway, and it worked fine.
>
> > and now here we do the second direct ide_complete_rq and here the rq is freed:
> >
> > BUG: unable to handle kernel NULL pointer dereference at
> > 0000000000000048
>
> I have just restored the original file to generate another crash
> dump. [the relevant part of] What I get is EIP == c0251311, edx == 0.
> This corresponds with the following machine code:
>
> c02512fc <ide_complete_rq>:
> c02512fc: 55 push %ebp
> c02512fd: 89 e5 mov %esp,%ebp
> c02512ff: 56 push %esi
> c0251300: 53 push %ebx
> c0251301: 83 ec 04 sub $0x4,%esp
> c0251304: 89 c3 mov %eax,%ebx
> c0251306: 89 d0 mov %edx,%eax
>
> /* ide_hwif_t *hwif = drive->hwif; */
> c0251308: 8b 73 28 mov 0x28(%ebx),%esi
>
> /* struct request *rq = hwif->rq; */
> c025130b: 8b 96 c8 01 00 00 mov 0x1c8(%esi),%edx
>
> /* if (blk_noretry_request(rq) .... */
> c0251311: f6 42 24 0e testb $0xe,0x24(%edx) /* !!! */
> c0251315: 74 04 je c025131b <ide_complete_rq+0x1f>
>
> blk_notretry_request is
>
> #define blk_noretry_request(rq) (blk_failfast_dev(rq) || \
> blk_failfast_transport(rq) || \
> blk_failfast_driver(rq))
>
> and
>
> #define blk_failfast_dev(rq) ((rq)->cmd_flags & REQ_FAILFAST_DEV)
> #define blk_failfast_transport(rq) ((rq)->cmd_flags & REQ_FAILFAST_TRANSPORT)
> #define blk_failfast_driver(rq) ((rq)->cmd_flags & REQ_FAILFAST_DRIVER)
>
> and
>
> #define REQ_FAILFAST_DEV (1 << __REQ_FAILFAST_DEV)
> #define REQ_FAILFAST_TRANSPORT (1 << __REQ_FAILFAST_TRANSPORT)
> #define REQ_FAILFAST_DRIVER (1 << __REQ_FAILFAST_DRIVER)
>
> and
>
> enum rq_flag_bits {
> __REQ_RW, /* not set, read. set, write */
> __REQ_FAILFAST_DEV, /* no driver retries of device errors */
> __REQ_FAILFAST_TRANSPORT, /* no driver retries of transport errors */
> __REQ_FAILFAST_DRIVER, /* no driver retries of driver errors */
> __REQ_DISCARD, /* request to discard sectors */
>
> [...]
>
> This means the values of the relevant __REQ_-constants are 1, 2, and
> 3 and (1 << 1) | (1 << 2) | (1 << 3) == 2 + 4 + 8 == 14 (== 0xe),
> hence testb $0xe, 0x24(%edx) (optimized by compiler). edx is 0.
> 0x24(%edx) is the field at offset 36 in a struct request, which is
> cmd_flags (on my computer).
>
> blk_end_io always returns zero for bio-less requests. More precisely,
> it calls end_that_request_data, which is
>
> static int end_that_request_data(struct request *rq, int error,
> unsigned int nr_bytes, unsigned int bidi_bytes)
> {
> if (rq->bio) {
> if (__end_that_request_first(rq, error, nr_bytes))
> return 1;
>
> /* Bidi request must be completed as a whole */
> if (blk_bidi_rq(rq) &&
> __end_that_request_first(rq->next_rq, error, bidi_bytes))
> return 1;
> }
>
> return 0;
> }
>
> ie it returns 0 for a request w/o a bio, and looks itself like this:
>
> static int blk_end_io(struct request *rq, int error, unsigned int nr_bytes,
> unsigned int bidi_bytes,
> int (drv_callback)(struct request *))
> {
> struct request_queue *q = rq->q;
> unsigned long flags = 0UL;
>
> if (end_that_request_data(rq, error, nr_bytes, bidi_bytes))
> return 1;
>
> /* Special feature for tricky drivers */
> if (drv_callback && drv_callback(rq))
> return 1;
>
> add_disk_randomness(rq->rq_disk);
>
> spin_lock_irqsave(q->queue_lock, flags);
> end_that_request_last(rq, error);
> spin_unlock_irqrestore(q->queue_lock, flags);
>
> return 0;
> }
>
> drv_callback is NULL and the 'final return value' is ultimatively
> returned from the ide_end_rq-call mentioned at the beginning of this
> overly long e-mail.
>
> An easy way to verify that would be a
>
> BUG_ON(!rq)
>
> inserted before the blkdev_noretry_request in ide_complete_rq (which I
> also did -- I have been doing this for long enough to never trust my
> own assumptions blindly ...).
>
> He who doesn't understand assembly will have a more difficult life
> because of that :-).
Well, this is a very good code analysis, I'd say, one for the books :)
> While this is interesting, my boss [righfully] hates it and I will now
> have to do at least an hour of additional overtime.
Sorry about that, same here :)
Here's another trace I did:
[ 266.037646] ide-cd: ide_cd_queue_pc: cmd[0]: 0x51, write: 0x0, timeout: 7000, cmd_flags: 0x8000
[ 266.037662] ide-cd: ide_cd_do_request: cmd: 0x51, block: 4294967295, marker: 114
[ 266.037667] ide_cd_do_request: dev hdc: type=a, flags=128a440
[ 266.037670] sector 4294967295, nr/cnr 0/0
[ 266.037675] bio dde1e840, biotail dde1e840, buffer (null), len 2
[ 266.037678] ide-cd: cdrom_do_block_pc: rq->cmd[0]: 0x51, rq->cmd_type: 0xa
[ 266.066559] ide-cd: cdrom_newpc_intr: cmd: 0x51, write: 0x0
[ 266.066567] ide-cd: cdrom_newpc_intr: DRQ: stat: 0x58, thislen: 2
[ 266.066570] ide-cd: ide_cd_check_ireason: ireason: 0x2, rw: 0x0
[ 266.066572] ide-cd: cdrom_newpc_intr: data transfer, rq->cmd_type: 0xa, ireason: 0x2
[ 266.149000] ide-cd: cdrom_newpc_intr: cmd: 0x51, write: 0x0
[ 266.149010] ide-cd: cdrom_newpc_intr: DRQ: stat: 0x50, thislen: 0
[ 266.149014] ide-cd: ide_cd_request_sense_fixup: rq->cmd[0]: 0x51
[ 266.149017] ide_complete_rq: completing rq, marker: 114
this is the end of the IRQ handler
[ 266.149023] __blk_put_request: marker: 114, ref_count: 2
[ 266.149033] ide_cd_queue_pc: putting rq, marker: 114
and here ide_cd_queue_pc comes _after_ that and frees the rq due to refcount ==
1.
[ 266.149039] __blk_put_request: marker: 114, ref_count: 1
[ 266.149045] blk_free_request: marker: 114, ref_count: 0
Now in the fragmented packet command case - that what you call 32/30 -
you'll have the first ide_complete_rq() call from ide_cd_error_cmd() and
the first decrement of the refcount. Had the rq had a bio, it'd never be
come to do a __blk_put_request and decrement the refcount.
But even if it did decrement it, as it does in the real case, that
wouldn't be a problem since the rq is still alive (refcount is now 1)
and it will be only freed in the second ide_complete_rq() at the end of
the IRQ handler.
But, it seems that ide_cd_queue_pc() goes after that first
ide_complete_rq() and decrements the refcount, as you suggest, right?
What bothers me here is that we're in IRQ context and running with
interrupts disabled so _actually_ the blk_put_request() part of
ide_cd_queue_pc() should be getting to run only _after_ the IRQ
handler is done and we should be getting the NULL ptr deref in
ide_cd_queue_pc(), but we don't. I guess I'm missing something here.
--
Regards/Gruss,
Boris.
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