[PATCH 1/1] mm: Fix UAF when anon vma name is used after vma is freed

[Suren Baghdasaryan <surenb@google.com>]

When adjacent vmas are being merged it can result in the vma that was
originally passed to madvise_update_vma being destroyed. In the current
implementation, the name parameter passed to madvise_update_vma points
directly to vma->anon_name->name and it is used after the call to
vma_merge. In the cases when vma_merge merges the original vma and
destroys it, this will result in use-after-free bug as shown below:

madvise_vma_behavior << passes vma->anon_name->name as name param
  madvise_update_vma(name)
    vma_merge
      __vma_adjust
        vm_area_free <-- frees the vma
    replace_vma_anon_name(name) <-- UAF

Fix this by passing madvise_update_vma a copy of the name.

Fixes: 9a10064f5625 ("mm: add a field to store names for private anonymous memory")
Reported-by: syzbot+aa7b3d4b35f9dc46a366@syzkaller.appspotmail.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/mm.h |  2 ++
 kernel/sys.c       |  1 -
 mm/madvise.c       | 14 +++++++++++++-
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 213cc569b192..e3490be76f35 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3370,6 +3370,8 @@ static inline int seal_check_future_write(int seals, struct vm_area_struct *vma)
 	return 0;
 }
 
+#define ANON_VMA_NAME_MAX_LEN	80
+
 #ifdef CONFIG_ANON_VMA_NAME
 int madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
 			  unsigned long len_in, const char *name);
diff --git a/kernel/sys.c b/kernel/sys.c
index ecc4cf019242..4cb6dc4bc09c 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2263,7 +2263,6 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
 
 #ifdef CONFIG_ANON_VMA_NAME
 
-#define ANON_VMA_NAME_MAX_LEN		80
 #define ANON_VMA_NAME_INVALID_CHARS	"\\`$[]"
 
 static inline bool is_valid_name_char(char ch)
diff --git a/mm/madvise.c b/mm/madvise.c
index 5604064df464..f36a5a9942d8 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -976,6 +976,8 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
 {
 	int error;
 	unsigned long new_flags = vma->vm_flags;
+	char name_buf[ANON_VMA_NAME_MAX_LEN];
+	const char *anon_name;
 
 	switch (behavior) {
 	case MADV_REMOVE:
@@ -1040,8 +1042,18 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
 		break;
 	}
 
+	anon_name = vma_anon_name(vma);
+	if (anon_name) {
+		/*
+		 * Make a copy of the name because vma might be destroyed when
+		 * merged with another one and the name parameter might be used
+		 * after that.
+		 */
+		strcpy(name_buf, anon_name);
+		anon_name = name_buf;
+	}
 	error = madvise_update_vma(vma, prev, start, end, new_flags,
-				   vma_anon_name(vma));
+				   anon_name);
 
 out:
 	/*
-- 
2.35.0.263.gb82422642f-goog