On 20/08/2021 22:31, Alyssa Rosenzweig wrote:

In lock_region, simplify the calculation of the region_width parameter. This field is the size, but encoded as log2(ceil(size)) - 1. log2(ceil(size)) may be computed directly as fls(size - 1). However, we want to use the 64-bit versions as the amount to lock can exceed 32-bits.

This avoids undefined behaviour when locking all memory (size ~0), caught by UBSAN.

It might have been useful to mention what it is that UBSAN specifically picked up (it took me a while to spot) - but anyway I think there's a bigger issue with it being completely wrong when size == ~0 (see below).

Signed-off-by: Alyssa Rosenzweig alyssa.rosenzweig@collabora.com Reported-and-tested-by: Chris Morgan macromorgan@hotmail.com Cc: stable@vger.kernel.org

However, I've confirmed this returns the same values and is certainly more simple, so:

Reviewed-by: Steven Price steven.price@arm.com

---

drivers/gpu/drm/panfrost/panfrost_mmu.c | 19 +++++-------------- 1 file changed, 5 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c index 0da5b3100ab1..f6e02d0392f4 100644 --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c @@ -62,21 +62,12 @@ static void lock_region(struct panfrost_device *pfdev, u32 as_nr, { u8 region_width; u64 region = iova & PAGE_MASK;

• /*

• * fls returns:
  

• * 1 .. 32
  

• *
  

• * 10 + fls(num_pages)
  

• * results in the range (11 .. 42)
  

• */
  

• size = round_up(size, PAGE_SIZE);

This seems to be the first issue - ~0 will be 'rounded up' to 0.

• region_width = 10 + fls(size >> PAGE_SHIFT);

fls(0) == 0, so region_width == 10.

• if ((size >> PAGE_SHIFT) != (1ul << (region_width - 11))) {

Presumably here's where UBSAN objects - we're shifting by a negative value, which even it it happens to works means the lock region is tiny and certainly not what was intended! It might well be worth a:

Fixes: f3ba91228e8e ("drm/panfrost: Add initial panfrost driver")

Note for anyone following along at (working-from-) home: although this code was cargo culted from kbase - kbase is fine because it takes a pfn and doesn't do the round_up() stage.

Which also exposes the second bug (fixed in patch 2): a size_t isn't big enough on 32 bit platforms (all Midgard/Bifrost GPUs have a VA size bigger than 32 bits). Again kbase gets away with a u32 because it's a pfn.

There is potentially a third bug which kbase only recently attempted to fix. The lock address is effectively rounded down by the hardware (the bottom bits are ignored). So if you have mask=(1<<region_width)-1 but (iova & mask) != ((iova + size) & mask) then you are potentially failing to lock the end of the intended region. kbase has added some code to handle this:

/* Round up if some memory pages spill into the next region. */ region_frame_number_start = pfn >> (lockaddr_size_log2 - PAGE_SHIFT); region_frame_number_end = (pfn + num_pages - 1) >> (lockaddr_size_log2 - PAGE_SHIFT);

if (region_frame_number_start < region_frame_number_end) lockaddr_size_log2 += 1;

I guess we should too?

Steve

• /* not pow2, so must go up to the next pow2 */
  

• region_width += 1;
  

• }

• /* The size is encoded as ceil(log2) minus(1), which may be calculated

• * with fls. The size must be clamped to hardware bounds.
  

• */
  

• size = max_t(u64, size, PAGE_SIZE);
• region_width = fls64(size - 1) - 1; region |= region_width;

/* Lock the region that needs to be updated */