diff options
authorLinus Torvalds <>2018-05-11 19:52:01 +0300
committerLinus Torvalds <>2018-05-11 19:52:01 +0300
commitbe83bbf806822b1b89e0a0f23cd87cddc409e429 (patch)
parent41e3e1082367221e99a59c8968a583706123ae04 (diff)
mmap: introduce sane default mmap limits
The internal VM "mmap()" interfaces are based on the mmap target doing everything using page indexes rather than byte offsets, because traditionally (ie 32-bit) we had the situation that the byte offset didn't fit in a register. So while the mmap virtual address was limited by the word size of the architecture, the backing store was not. So we're basically passing "pgoff" around as a page index, in order to be able to describe backing store locations that are much bigger than the word size (think files larger than 4GB etc). But while this all makes a ton of sense conceptually, we've been dogged by various drivers that don't really understand this, and internally work with byte offsets, and then try to work with the page index by turning it into a byte offset with "pgoff << PAGE_SHIFT". Which obviously can overflow. Adding the size of the mapping to it to get the byte offset of the end of the backing store just exacerbates the problem, and if you then use this overflow-prone value to check various limits of your device driver mmap capability, you're just setting yourself up for problems. The correct thing for drivers to do is to do their limit math in page indices, the way the interface is designed. Because the generic mmap code _does_ test that the index doesn't overflow, since that's what the mmap code really cares about. HOWEVER. Finding and fixing various random drivers is a sisyphean task, so let's just see if we can just make the core mmap() code do the limiting for us. Realistically, the only "big" backing stores we need to care about are regular files and block devices, both of which are known to do this properly, and which have nice well-defined limits for how much data they can access. So let's special-case just those two known cases, and then limit other random mmap users to a backing store that still fits in "unsigned long". Realistically, that's not much of a limit at all on 64-bit, and on 32-bit architectures the only worry might be the GPU drivers, which can have big physical address spaces. To make it possible for drivers like that to say that they are 64-bit clean, this patch does repurpose the "FMODE_UNSIGNED_OFFSET" bit in the file flags to allow drivers to mark their file descriptors as safe in the full 64-bit mmap address space. [ The timing for doing this is less than optimal, and this should really go in a merge window. But realistically, this needs wide testing more than it needs anything else, and being main-line is the only way to do that. So the earlier the better, even if it's outside the proper development cycle - Linus ] Cc: Kees Cook <> Cc: Dan Carpenter <> Cc: Al Viro <> Cc: Willy Tarreau <> Cc: Dave Airlie <> Signed-off-by: Linus Torvalds <>
1 files changed, 32 insertions, 0 deletions
diff --git a/mm/mmap.c b/mm/mmap.c
index 9d5968d1e8e3..6fc435760086 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1324,6 +1324,35 @@ static inline int mlock_future_check(struct mm_struct *mm,
return 0;
+static inline u64 file_mmap_size_max(struct file *file, struct inode *inode)
+ if (S_ISREG(inode->i_mode))
+ return inode->i_sb->s_maxbytes;
+ if (S_ISBLK(inode->i_mode))
+ /* Special "we do even unsigned file positions" case */
+ if (file->f_mode & FMODE_UNSIGNED_OFFSET)
+ return 0;
+ /* Yes, random drivers might want more. But I'm tired of buggy drivers */
+ return ULONG_MAX;
+static inline bool file_mmap_ok(struct file *file, struct inode *inode,
+ unsigned long pgoff, unsigned long len)
+ u64 maxsize = file_mmap_size_max(file, inode);
+ if (maxsize && len > maxsize)
+ return false;
+ maxsize -= len;
+ if (pgoff > maxsize >> PAGE_SHIFT)
+ return false;
+ return true;
* The caller must hold down_write(&current->mm->mmap_sem).
@@ -1409,6 +1438,9 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
struct inode *inode = file_inode(file);
unsigned long flags_mask;
+ if (!file_mmap_ok(file, inode, pgoff, len))
+ return -EOVERFLOW;
flags_mask = LEGACY_MAP_MASK | file->f_op->mmap_supported_flags;
switch (flags & MAP_TYPE) {