[PATCH v2 0/3] fixes for udmabuf (memfd sealing checks and a leak)

[Jann Horn <jannh@xxxxxxxxxx>]

I have tested that patches 2 and 3 work using the following reproducers.
I did not write a reproducer for the issue described in patch 1.

Reproducer for F_SEAL_FUTURE_WRITE not being respected:
```
#define _GNU_SOURCE
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <linux/udmabuf.h>

#define SYSCHK(x) ({          \
  typeof(x) __res = (x);      \
  if (__res == (typeof(x))-1) \
    err(1, "SYSCHK(" #x ")"); \
  __res;                      \
})

int main(void) {
  int memfd = SYSCHK(memfd_create("test", MFD_ALLOW_SEALING));
  SYSCHK(ftruncate(memfd, 0x1000));
  SYSCHK(fcntl(memfd, F_ADD_SEALS, F_SEAL_SHRINK|F_SEAL_FUTURE_WRITE));
  int udmabuf_fd = SYSCHK(open("/dev/udmabuf", O_RDWR));
  struct udmabuf_create create_arg = {
    .memfd = memfd,
    .flags = 0,
    .offset = 0,
    .size = 0x1000
  };
  int buf_fd = SYSCHK(ioctl(udmabuf_fd, UDMABUF_CREATE, &create_arg));
  printf("created udmabuf buffer fd %d\n", buf_fd);
  char *map = SYSCHK(mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, buf_fd, 0));
  *map = 'a';
}
```

Reproducer for the memory leak (if you run this for a while, your memory
usage will steadily go up, and /sys/kernel/debug/dma_buf/bufinfo will
contain a ton of entries):
```
#define _GNU_SOURCE
#include <err.h>
#include <errno.h>
#include <assert.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/resource.h>
#include <linux/udmabuf.h>

#define SYSCHK(x) ({          \
  typeof(x) __res = (x);      \
  if (__res == (typeof(x))-1) \
    err(1, "SYSCHK(" #x ")"); \
  __res;                      \
})

int main(void) {
  int memfd = SYSCHK(memfd_create("test", MFD_ALLOW_SEALING));
  SYSCHK(ftruncate(memfd, 0x1000));
  SYSCHK(fcntl(memfd, F_ADD_SEALS, F_SEAL_SHRINK));
  int udmabuf_fd = SYSCHK(open("/dev/udmabuf", O_RDWR));

  // prevent creating new FDs
  struct rlimit rlim = { .rlim_cur = 1, .rlim_max = 1 };
  SYSCHK(setrlimit(RLIMIT_NOFILE, &rlim));

  while (1) {
    struct udmabuf_create create_arg = {
      .memfd = memfd,
      .flags = 0,
      .offset = 0,
      .size = 0x1000
    };
    int buf_fd = ioctl(udmabuf_fd, UDMABUF_CREATE, &create_arg);
    assert(buf_fd == -1);
    assert(errno == EMFILE);
  }
}
```

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
Changes in v2:
- patch 1/3: use file_inode instead of ->f_inode (Vivek)
- patch 1/3: add comment explaining the inode_lock_shared()
- patch 3/3: remove unused parameter (Vivek)
- patch 3/3: add comment on cleanup (Vivek)
- collect Acks
- Link to v1: https://lore.kernel.org/r/20241203-udmabuf-fixes-v1-0-f99281c345aa@xxxxxxxxxx

---
Jann Horn (3):
      udmabuf: fix racy memfd sealing check
      udmabuf: also check for F_SEAL_FUTURE_WRITE
      udmabuf: fix memory leak on last export_udmabuf() error path

 drivers/dma-buf/udmabuf.c | 43 +++++++++++++++++++++++++++----------------
 1 file changed, 27 insertions(+), 16 deletions(-)
---
base-commit: b86545e02e8c22fb89218f29d381fa8e8b91d815
change-id: 20241203-udmabuf-fixes-d0435ebab663

-- 
Jann Horn <jannh@xxxxxxxxxx>