From d3ed3a40951c964d30dc7fc63d13e87a873ea300 Mon Sep 17 00:00:00 2001 From: "Lorenzo Stoakes (ARM)" Date: Thu, 24 Sep 2026 15:48:24 +0100 Subject: [PATCH] drivers/char/mem: mmap readonly MAP_SHARED-/dev/zero correctly Rather surprisingly, opening /dev/zero read-only then mmap()'ing it MAP_SHARED gets you true anonymous memory (albeit in a VMA with non-NULL vma->vm_file). This is a by-product of MAP_PRIVATE-/dev/zero being how anonymous memory was mapped in Linux's distant past. It happens because mmap_zero_prepare() gates on VMA_SHARED_BIT and when mapping a read-only file MAP_SHARED, do_mmap() clears VMA_SHARED_BIT and VMA_MAYWRITE_BIT. The gating is incorrect - the (poorly named) VMA_MAYSHARE_BIT flag exists explicitly to tell you if something was originally mapped MAP_SHARED. So the fix is simple - gate on this instead. This isn't exactly a common use case, but it's unexpected behaviour which now causes an assert if CONFIG_DEBUG_VM is set. While this bug has existed since the dawn of time for linux (or at least since 2.6.12), it hasn't caused issues in the past, so while it's incorrect behaviour, it doesn't seem necessary to backport that far. The mapping is now accounted at mmap time and can fail with -ENOMEM under strict overcommit, and read faults allocate folios. However this is normal behaviour for a read-only shmem mapping. Commit 93c0c8dc87f6 ("mm/rmap: use anon pgoff to track MAP_PRIVATE file-backed anon folios") is the first patch at which the debug assert fires, so target that instead. Link: https://lore.kernel.org/20260924-fix-dev-zero-readonly-shared-v1-1-153c2111e323@kernel.org Fixes: 93c0c8dc87f6 ("mm/rmap: use anon pgoff to track MAP_PRIVATE file-backed anon folios") Signed-off-by: Lorenzo Stoakes (ARM) Signed-off-by: Andrew Morton Reported-by: Closes: https://lore.kernel.org/linux-mm/6ab4ae75.80e1c6cc.1e8e5f.000d.GAE@google.com/ Acked-by: David Hildenbrand (Arm) Cc: Liam R. Howlett Cc: Vlastimil Babka Cc: Jann Horn Cc: Pedro Falcato Cc: Mike Rapoport Cc: Suren Baghdasaryan Cc: Michal Hocko Cc: Arnd Bergmann Cc: Greg Kroah-Hartman Cc: Lance Yang --- drivers/char/mem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/char/mem.c b/drivers/char/mem.c index 63253d1de5d7..5b93c92c2cf1 100644 --- a/drivers/char/mem.c +++ b/drivers/char/mem.c @@ -503,7 +503,7 @@ static int mmap_zero_prepare(struct vm_area_desc *desc) #ifndef CONFIG_MMU return -ENOSYS; #endif - if (vma_desc_test(desc, VMA_SHARED_BIT)) + if (vma_desc_test(desc, VMA_MAYSHARE_BIT)) return shmem_zero_setup_desc(desc); /*