Re: [PATCH] mm/mmap: undo ->mmap() when arch_validate_flags() fails

On Thu, Sep 29, 2022 at 5:51 PM Carlos Llamas cmllamas@google.com wrote:

Commit c462ac288f2c ("mm: Introduce arch_validate_flags()") added a late check in mmap_region() to let architectures validate vm_flags. The check needs to happen after calling ->mmap() as the flags can potentially be modified during this callback.

If arch_validate_flags() check fails we unmap and free the vma. However, the error path fails to undo the ->mmap() call that previously succeeded and depending on the specific ->mmap() implementation this translates to reference increments, memory allocations and other operations what will not be cleaned up.

There are several places (mainly device drivers) where this is an issue. However, one specific example is bpf_map_mmap() which keeps count of the mappings in map->writecnt. The count is incremented on ->mmap() and then decremented on vm_ops->close(). When arch_validate_flags() fails this count is off since bpf_map_mmap_close() is never called.

One can reproduce this issue in arm64 devices with MTE support. Here the vm_flags are checked to only allow VM_MTE if VM_MTE_ALLOWED has been set previously. From userspace then is enough to pass the PROT_MTE flag to mmap() syscall to trigger the arch_validate_flags() failure.

The following program reproduces this issue:

#include <stdio.h> #include <unistd.h> #include <linux/unistd.h> #include <linux/bpf.h> #include <sys/mman.h>

int main(void) { union bpf_attr attr = { .map_type = BPF_MAP_TYPE_ARRAY, .key_size = sizeof(int), .value_size = sizeof(long long), .max_entries = 256, .map_flags = BPF_F_MMAPABLE, }; int fd;

    fd = syscall(__NR_bpf, BPF_MAP_CREATE, &attr, sizeof(attr));
    mmap(NULL, 4096, PROT_WRITE | PROT_MTE, MAP_SHARED, fd, 0);

    return 0;

}

By manually adding some log statements to the vm_ops callbacks we can confirm that when passing PROT_MTE to mmap() the map->writecnt is off upon ->release():

With PROT_MTE flag: root@debian:~# ./bpf-test [ 111.263874] bpf_map_write_active_inc: map=9 writecnt=1 [ 111.288763] bpf_map_release: map=9 writecnt=1

Without PROT_MTE flag: root@debian:~# ./bpf-test [ 157.816912] bpf_map_write_active_inc: map=10 writecnt=1 [ 157.830442] bpf_map_write_active_dec: map=10 writecnt=0 [ 157.832396] bpf_map_release: map=10 writecnt=0

This patch fixes the above issue by calling vm_ops->close() when the arch_validate_flags() check fails, after this we can proceed to unmap and free the vma on the error path.

Fixes: c462ac288f2c ("mm: Introduce arch_validate_flags()") Cc: Catalin Marinas catalin.marinas@arm.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Suren Baghdasaryan surenb@google.com Cc: stable@vger.kernel.org # v5.10+ Signed-off-by: Carlos Llamas cmllamas@google.com

---

Makes sense to me, open/close callbacks should be symmetrical. From BPF-side of things:

Acked-by: Andrii Nakryiko andrii@kernel.org

mm/mmap.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/mmap.c b/mm/mmap.c index 9d780f415be3..36c08e2c78da 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1797,7 +1797,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr, if (!arch_validate_flags(vma->vm_flags)) { error = -EINVAL; if (file)

•                   goto unmap_and_free_vma;
  

•                   goto close_and_free_vma;
            else
                    goto free_vma;
    }
  

@@ -1844,6 +1844,9 @@ unsigned long mmap_region(struct file *file, unsigned long addr,

    return addr;

+close_and_free_vma:

•   if (vma->vm_ops && vma->vm_ops->close)
  

•           vma->vm_ops->close(vma);
  

unmap_and_free_vma: fput(vma->vm_file); vma->vm_file = NULL; -- 2.38.0.rc1.362.ged0d419d3c-goog