[PATCH] drm/panthor: Be stricter about IO mapping flags

[Jann Horn]

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 at vger.kernel.org
Fixes: 5fe909cae118 ("drm/panthor: Add the device logical block")
Signed-off-by: Jann Horn <jannh at 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(+)

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

-- 
Jann Horn <jannh at google.com>