| Message ID | 20241105-panthor-flush-page-fixes-v1-1-829aaf37db93@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | drm/panthor: Be stricter about IO mapping flags | expand |
On Tue, 05 Nov 2024 00:17:13 +0100 Jann Horn <jannh@google.com> wrote: > The current panthor_device_mmap_io() implementation has two issues: > > 1. For mapping DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET, > panthor_device_mmap_io() bails if VM_WRITE is set, but does not clear > VM_MAYWRITE. That means userspace can use mprotect() to make the mapping > writable later on. This is a classic Linux driver gotcha. > I don't think this actually has any impact in practice: > When the GPU is powered, writes to the FLUSH_ID seem to be ignored; and > when the GPU is not powered, the dummy_latest_flush page provided by the > driver is deliberately designed to not do any flushes, so the only thing > writing to the dummy_latest_flush could achieve would be to make *more* > flushes happen. > > 2. panthor_device_mmap_io() does not block MAP_PRIVATE mappings (which are > mappings without the VM_SHARED flag). > MAP_PRIVATE in combination with VM_MAYWRITE indicates that the VMA has > copy-on-write semantics, which for VM_PFNMAP are semi-supported but > fairly cursed. > In particular, in such a mapping, the driver can only install PTEs > during mmap() by calling remap_pfn_range() (because remap_pfn_range() > wants to **store the physical address of the mapped physical memory into > the vm_pgoff of the VMA**); installing PTEs later on with a fault > handler (as panthor does) is not supported in private mappings, and so > if you try to fault in such a mapping, vmf_insert_pfn_prot() splats when > it hits a BUG() check. > > Fix it by clearing the VM_MAYWRITE flag (userspace writing to the FLUSH_ID > doesn't make sense) and requiring VM_SHARED (copy-on-write semantics for > the FLUSH_ID don't make sense). > > Reproducers for both scenarios are in the notes of my patch on the mailing > list; I tested that these bugs exist on a Rock 5B machine. > > Note that I only compile-tested the patch, I haven't tested it; I don't > have a working kernel build setup for the test machine yet. Please test it > before applying it. Sure, I'll test it before applying. > > Cc: stable@vger.kernel.org > Fixes: 5fe909cae118 ("drm/panthor: Add the device logical block") > Signed-off-by: Jann Horn <jannh@google.com> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com> > --- > First testcase (can write to the FLUSH_ID): > > ``` > > typeof(x) __res = (x); \ > if (__res == (typeof(x))-1) \ > err(1, "SYSCHK(" #x ")"); \ > __res; \ > }) > > int main(void) { > int fd = SYSCHK(open(GPU_PATH, O_RDWR)); > > // sanity-check that PROT_WRITE+MAP_SHARED fails > void *mmap_write_res = mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, > MAP_SHARED, fd, DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET); > if (mmap_write_res == MAP_FAILED) { > perror("mmap() with PROT_WRITE+MAP_SHARED failed as expected"); > } else { > errx(1, "mmap() with PROT_WRITE+MAP_SHARED worked???"); > } > > // make a PROT_READ+MAP_SHARED mapping, and upgrade it to writable > void *mmio_page = SYSCHK(mmap(NULL, 0x1000, PROT_READ, MAP_SHARED, > fd, DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET)); > SYSCHK(mprotect(mmio_page, 0x1000, PROT_READ|PROT_WRITE)); > > volatile uint32_t *flush_counter = (volatile uint32_t*)mmio_page; > > uint32_t last_old = -1; > while (1) { > uint32_t old_val = *flush_counter; > *flush_counter = 1111; > uint32_t new_val = *flush_counter; > if (old_val != last_old) > printf("flush counter: old=%u, new=%u\n", old_val, new_val); > last_old = old_val; > } > } > ``` > > Second testcase (triggers BUG() splat): > ``` > > typeof(x) __res = (x); \ > if (__res == (typeof(x))-1) \ > err(1, "SYSCHK(" #x ")"); \ > __res; \ > }) > > int main(void) { > int fd = SYSCHK(open(GPU_PATH, O_RDWR)); > > // make a PROT_READ+**MAP_PRIVATE** mapping > void *ptr = SYSCHK(mmap(NULL, 0x1000, PROT_READ, MAP_PRIVATE, > fd, DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET)); > > // trigger a read fault > *(volatile char *)ptr; > } > ``` > > The second testcase splats like this: > ``` > [ 2918.411814] ------------[ cut here ]------------ > [ 2918.411857] kernel BUG at mm/memory.c:2220! > [ 2918.411955] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP > [...] > [ 2918.416147] CPU: 3 PID: 2934 Comm: private_user_fl Tainted: G O 6.1.43-19-rk2312 #428a0a5e6 > [ 2918.417043] Hardware name: Radxa ROCK 5B (DT) > [ 2918.417464] pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) > [ 2918.418119] pc : vmf_insert_pfn_prot+0x40/0xe4 > [ 2918.418567] lr : panthor_mmio_vm_fault+0xb0/0x12c [panthor] > [...] > [ 2918.425746] Call trace: > [ 2918.425972] vmf_insert_pfn_prot+0x40/0xe4 > [ 2918.426342] __do_fault+0x38/0x7c > [ 2918.426648] __handle_mm_fault+0x404/0x6dc > [ 2918.427018] handle_mm_fault+0x13c/0x18c > [ 2918.427374] do_page_fault+0x194/0x33c > [ 2918.427716] do_translation_fault+0x60/0x7c > [ 2918.428095] do_mem_abort+0x44/0x90 > [ 2918.428410] el0_da+0x40/0x68 > [ 2918.428685] el0t_64_sync_handler+0x9c/0xf8 > [ 2918.429067] el0t_64_sync+0x174/0x178 > ``` > --- > drivers/gpu/drm/panthor/panthor_device.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/drivers/gpu/drm/panthor/panthor_device.c b/drivers/gpu/drm/panthor/panthor_device.c > index 4082c8f2951dfdace7f73a24d6fe34e9e7f920eb..6fbff516c1c1f047fcb4dee17b87d8263616dc0c 100644 > --- a/drivers/gpu/drm/panthor/panthor_device.c > +++ b/drivers/gpu/drm/panthor/panthor_device.c > @@ -390,11 +390,15 @@ int panthor_device_mmap_io(struct panthor_device *ptdev, struct vm_area_struct * > { > u64 offset = (u64)vma->vm_pgoff << PAGE_SHIFT; > > + if ((vma->vm_flags & VM_SHARED) == 0) > + return -EINVAL; > + > switch (offset) { > case DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET: > if (vma->vm_end - vma->vm_start != PAGE_SIZE || > (vma->vm_flags & (VM_WRITE | VM_EXEC))) > return -EINVAL; > + vm_flags_clear(vma, VM_MAYWRITE); > > break; > > > --- > base-commit: d78f0ee0406803cda8801fd5201746ccf89e5e4a > change-id: 20241104-panthor-flush-page-fixes-fe4202bb18c0 >
diff --git a/drivers/gpu/drm/panthor/panthor_device.c b/drivers/gpu/drm/panthor/panthor_device.c index 4082c8f2951dfdace7f73a24d6fe34e9e7f920eb..6fbff516c1c1f047fcb4dee17b87d8263616dc0c 100644 --- a/drivers/gpu/drm/panthor/panthor_device.c +++ b/drivers/gpu/drm/panthor/panthor_device.c @@ -390,11 +390,15 @@ int panthor_device_mmap_io(struct panthor_device *ptdev, struct vm_area_struct * { u64 offset = (u64)vma->vm_pgoff << PAGE_SHIFT; + if ((vma->vm_flags & VM_SHARED) == 0) + return -EINVAL; + switch (offset) { case DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET: if (vma->vm_end - vma->vm_start != PAGE_SIZE || (vma->vm_flags & (VM_WRITE | VM_EXEC))) return -EINVAL; + vm_flags_clear(vma, VM_MAYWRITE); break;
The current panthor_device_mmap_io() implementation has two issues: 1. For mapping DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET, panthor_device_mmap_io() bails if VM_WRITE is set, but does not clear VM_MAYWRITE. That means userspace can use mprotect() to make the mapping writable later on. This is a classic Linux driver gotcha. I don't think this actually has any impact in practice: When the GPU is powered, writes to the FLUSH_ID seem to be ignored; and when the GPU is not powered, the dummy_latest_flush page provided by the driver is deliberately designed to not do any flushes, so the only thing writing to the dummy_latest_flush could achieve would be to make *more* flushes happen. 2. panthor_device_mmap_io() does not block MAP_PRIVATE mappings (which are mappings without the VM_SHARED flag). MAP_PRIVATE in combination with VM_MAYWRITE indicates that the VMA has copy-on-write semantics, which for VM_PFNMAP are semi-supported but fairly cursed. In particular, in such a mapping, the driver can only install PTEs during mmap() by calling remap_pfn_range() (because remap_pfn_range() wants to **store the physical address of the mapped physical memory into the vm_pgoff of the VMA**); installing PTEs later on with a fault handler (as panthor does) is not supported in private mappings, and so if you try to fault in such a mapping, vmf_insert_pfn_prot() splats when it hits a BUG() check. Fix it by clearing the VM_MAYWRITE flag (userspace writing to the FLUSH_ID doesn't make sense) and requiring VM_SHARED (copy-on-write semantics for the FLUSH_ID don't make sense). Reproducers for both scenarios are in the notes of my patch on the mailing list; I tested that these bugs exist on a Rock 5B machine. Note that I only compile-tested the patch, I haven't tested it; I don't have a working kernel build setup for the test machine yet. Please test it before applying it. Cc: stable@vger.kernel.org Fixes: 5fe909cae118 ("drm/panthor: Add the device logical block") Signed-off-by: Jann Horn <jannh@google.com> --- First testcase (can write to the FLUSH_ID): ``` typeof(x) __res = (x); \ if (__res == (typeof(x))-1) \ err(1, "SYSCHK(" #x ")"); \ __res; \ }) int main(void) { int fd = SYSCHK(open(GPU_PATH, O_RDWR)); // sanity-check that PROT_WRITE+MAP_SHARED fails void *mmap_write_res = mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, fd, DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET); if (mmap_write_res == MAP_FAILED) { perror("mmap() with PROT_WRITE+MAP_SHARED failed as expected"); } else { errx(1, "mmap() with PROT_WRITE+MAP_SHARED worked???"); } // make a PROT_READ+MAP_SHARED mapping, and upgrade it to writable void *mmio_page = SYSCHK(mmap(NULL, 0x1000, PROT_READ, MAP_SHARED, fd, DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET)); SYSCHK(mprotect(mmio_page, 0x1000, PROT_READ|PROT_WRITE)); volatile uint32_t *flush_counter = (volatile uint32_t*)mmio_page; uint32_t last_old = -1; while (1) { uint32_t old_val = *flush_counter; *flush_counter = 1111; uint32_t new_val = *flush_counter; if (old_val != last_old) printf("flush counter: old=%u, new=%u\n", old_val, new_val); last_old = old_val; } } ``` Second testcase (triggers BUG() splat): ``` typeof(x) __res = (x); \ if (__res == (typeof(x))-1) \ err(1, "SYSCHK(" #x ")"); \ __res; \ }) int main(void) { int fd = SYSCHK(open(GPU_PATH, O_RDWR)); // make a PROT_READ+**MAP_PRIVATE** mapping void *ptr = SYSCHK(mmap(NULL, 0x1000, PROT_READ, MAP_PRIVATE, fd, DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET)); // trigger a read fault *(volatile char *)ptr; } ``` The second testcase splats like this: ``` [ 2918.411814] ------------[ cut here ]------------ [ 2918.411857] kernel BUG at mm/memory.c:2220! [ 2918.411955] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP [...] [ 2918.416147] CPU: 3 PID: 2934 Comm: private_user_fl Tainted: G O 6.1.43-19-rk2312 #428a0a5e6 [ 2918.417043] Hardware name: Radxa ROCK 5B (DT) [ 2918.417464] pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) [ 2918.418119] pc : vmf_insert_pfn_prot+0x40/0xe4 [ 2918.418567] lr : panthor_mmio_vm_fault+0xb0/0x12c [panthor] [...] [ 2918.425746] Call trace: [ 2918.425972] vmf_insert_pfn_prot+0x40/0xe4 [ 2918.426342] __do_fault+0x38/0x7c [ 2918.426648] __handle_mm_fault+0x404/0x6dc [ 2918.427018] handle_mm_fault+0x13c/0x18c [ 2918.427374] do_page_fault+0x194/0x33c [ 2918.427716] do_translation_fault+0x60/0x7c [ 2918.428095] do_mem_abort+0x44/0x90 [ 2918.428410] el0_da+0x40/0x68 [ 2918.428685] el0t_64_sync_handler+0x9c/0xf8 [ 2918.429067] el0t_64_sync+0x174/0x178 ``` --- drivers/gpu/drm/panthor/panthor_device.c | 4 ++++ 1 file changed, 4 insertions(+) --- base-commit: d78f0ee0406803cda8801fd5201746ccf89e5e4a change-id: 20241104-panthor-flush-page-fixes-fe4202bb18c0