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

[Carlos Llamas <cmllamas@google.com>]

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>
---
 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