Re: [RFC PATCH 1/1] dma-debug: fix check_for_illegal_area() in debug_dma_map_sg()

[Robin Murphy]

On 2021-07-06 20:12, Gerald Schaefer wrote:
> On Tue, 6 Jul 2021 10:22:40 +0100
> Robin Murphy <robin.murphy@xxxxxxx> wrote:
>
> > On 2021-07-05 19:52, Gerald Schaefer wrote:
> > > The following warning occurred sporadically on s390:
> > > DMA-API: nvme 0006:00:00.0: device driver maps memory from kernel text or rodata [addr=0000000048cc5e2f] [len=131072]
> > > WARNING: CPU: 4 PID: 825 at kernel/dma/debug.c:1083 check_for_illegal_area+0xa8/0x138
> > >
> > > It is a false-positive warning, due to a broken logic in debug_dma_map_sg().
> > > check_for_illegal_area() should check for overlay of sg elements with kernel
> > > text or rodata. It is called with sg_dma_len(s) instead of s->length as
> > > parameter. After the call to ->map_sg(), sg_dma_len() contains the length
> > > of possibly combined sg elements in the DMA address space, and not the
> > > individual sg element length, which would be s->length.
> > >
> > > The check will then use the kernel start address of an sg element, and add
> > > the DMA length for overlap check, which can result in the false-positive
> > > warning because the DMA length can be larger than the actual single sg
> > > element length in kernel address space.
> > >
> > > In addition, the call to check_for_illegal_area() happens in the iteration
> > > over mapped_ents, which will not include all individual sg elements if
> > > any of them were combined in ->map_sg().
> > >
> > > Fix this by using s->length instead of sg_dma_len(s). Also put the call to
> > > check_for_illegal_area() in a separate loop, iterating over all the
> > > individual sg elements ("nents" instead of "mapped_ents").
> > >
> > > Fixes: 884d05970bfb ("dma-debug: use sg_dma_len accessor")
> > > Tested-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
> > > Signed-off-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
> > > ---
> > >    kernel/dma/debug.c | 10 ++++++----
> > >    1 file changed, 6 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
> > > index 14de1271463f..d7d44b7fe7e2 100644
> > > --- a/kernel/dma/debug.c
> > > +++ b/kernel/dma/debug.c
> > > @@ -1299,6 +1299,12 @@ void debug_dma_map_sg(struct device *dev, struct scatterlist *sg,
> > >    	if (unlikely(dma_debug_disabled()))
> > >    		return;
> > >    
> > > +	for_each_sg(sg, s, nents, i) {
> > > +		if (!PageHighMem(sg_page(s))) {
> > > +			check_for_illegal_area(dev, sg_virt(s), s->length);
> > > +		}
> > > +	}
> > > +
> > >    	for_each_sg(sg, s, mapped_ents, i) {
> > >    		entry = dma_entry_alloc();
> > >    		if (!entry)
> > > @@ -1316,10 +1322,6 @@ void debug_dma_map_sg(struct device *dev, struct scatterlist *sg,
> > >    
> > >    		check_for_stack(dev, sg_page(s), s->offset);
> >
> > Strictly this should probably be moved to the new loop as well, as it is
> > similarly concerned with validating the source segments rather than the
> > DMA mappings - I think with virtually-mapped stacks it might technically
> > be possible for a stack page to be physically adjacent to a "valid" page
> > such that it could get merged and overlooked if it were near the end of
> > the list, although in fairness that would probably be indicative of
> > something having gone far more fundamentally wrong. Otherwise, the
> > overall reasoning looks sound to me.
>
> I see, good point. I think I can add this to my patch, and a different
> subject like "dma-debug: fix sg checks in debug_dma_map_sg()".

TBH it's more of a conceptual cleanliness thing than a significant 
practical concern, but if we *are* breaking out a separate "validate the 
source elements" step then it does seem logical to capture everything 
relevant at once.

> However, I do not quite understand why check_for_stack() does not also
> consider s->length. It seems to check only the first page of an sg
> element.
>
> So, shouldn't check_for_stack() behave similar to check_for_illegal_area(),
> i.e. check all source sg elements for overlap with the task stack area?

Realistically, creating a scatterlist segment pointing to the stack at 
all would already be quite an audacious feat of brokenness, but getting 
a random stack page in the middle of a segment would seem to imply 
something having gone so catastrophically wrong that it's destined to 
end very badly whether or not dma-debug squawks about it - not to 
mention getting lucky enough for said random stack page to actually 
belong to the current task stack in the first place :)

Robin.

> If yes, then this probably should be a separate patch, but I can try
> to come up with something and send a new RFC with two patches. Maybe
> check_for_stack() can also be integrated into check_for_illegal_area(),
> they are both called at the same places. And mapping memory from the
> stack also sounds rather illegal.