On Tue, Aug 24, 2021 at 12:30 PM Alyssa Rosenzweig alyssa.rosenzweig@collabora.com wrote:
Chris Morgan reported UBSAN errors in panfrost and tracked them down to the size computation in lock_region. This calculation is overcomplicated (cargo culted from kbase) and can be simplified with kernel helpers and some mathematical identities. The first patch in the series rewrites the calculation in a form avoiding undefined behaviour; Chris confirms it placates UBSAN.
While researching this function, I noticed a pair of other potential bugs: Bifrost can lock more than 4GiB at a time, but must lock at least 32KiB at a time. The latter patches in the series handle these cases.
In review of v1 of this series, Steven pointed out a fourth potential bug: rounding down the iova can truncate the lock region. v2 adds a new patch for this case.
The size computation was unit-tested in userspace. Relevant code below, just missing some copypaste definitions for fls64/clamp/etc:
#define MIN_LOCK (1ULL << 12) #define MAX_LOCK (1ULL << 48) struct { uint64_t size; uint8_t encoded; } tests[] = { /* Clamping */ { 0, 11 }, { 1, 11 }, { 2, 11 }, { 4095, 11 }, /* Power of two */ { 4096, 11 }, /* Round up */ { 4097, 12 }, { 8192, 12 }, { 16384, 13 }, { 16385, 14 }, /* Maximum */ { ~0ULL, 47 }, }; static uint8_t region_width(uint64_t size) { size = clamp(size, MIN_LOCK, MAX_LOCK); return fls64(size - 1) - 1; } int main(int argc, char **argv) { for (unsigned i = 0; i < ARRAY_SIZE(tests); ++i) { uint64_t test = tests[i].size; uint8_t expected = tests[i].encoded; uint8_t actual = region_width(test); assert(expected == actual); } }Changes in v2:
- New patch for non-aligned lock addresses
- Commit message improvements.
- Add Steven's tags.
Alyssa Rosenzweig (4): drm/panfrost: Simplify lock_region calculation drm/panfrost: Use u64 for size in lock_region drm/panfrost: Clamp lock region to Bifrost minimum drm/panfrost: Handle non-aligned lock addresses
drivers/gpu/drm/panfrost/panfrost_mmu.c | 32 ++++++++++-------------- drivers/gpu/drm/panfrost/panfrost_regs.h | 2 ++ 2 files changed, 15 insertions(+), 19 deletions(-)
For the series,
Reviewed-by: Rob Herring robh@kernel.org