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Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , David Hildenbrand , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Arnd Bergmann , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lance Yang , syzbot+c181d3198e98f8aef8b9@syzkaller.appspotmail.com Subject: Re: [PATCH] drivers/char/mem: mmap readonly MAP_SHARED-/dev/zero correctly Message-ID: <2026092406-irritably-maggot-9512@gregkh> References: <20260924-fix-dev-zero-readonly-shared-v1-1-153c2111e323@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260924-fix-dev-zero-readonly-shared-v1-1-153c2111e323@kernel.org> X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 2D49318000A X-Stat-Signature: j9prex9j9fkrtxy9c69hwtydchq4pdb6 X-HE-Tag: 1790262547-945002 X-HE-Meta: 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 NOdVQSMU jfrIhAldcMdL/fcyuLN6wUhst5WrgMqP1O8lunLnepWeu84J1bCmhf2gQR8SLTlbcqrS3QA9Lv29ihiStrYmGcicIlVesY5f0J4s702WpAkAZWL8L+uXJDA9B+P0xXNF/igHzzcoztqoY8OY1l6+kNkfpn1LR8Zyl0nW3EWgiIpRaReriHVhfElyl0KozkX+RcXuN0/ZLNOdLS/UakzfZ31Up6N4J9rIdgi593SP4n+yU4qHhTqOqpskBeY6XowLfJ44yF9tT3LxjTQ/Tu8dCq1uDi0HfIlrV4x+TjvVyY3Ed8ek4sr3xXzoyaT78f93i3dzAOpLJZruk74cMKZVynzELZe349uzWVpX9fqoRxkALWQYYqbs6A40qAspYsVF8UHStq7opK+MGZDGRvmSRj2gPp8Auv5le/Ek2jQw0WePqFmpklk4Cxdyltxca9OQjqqvLI7TiC0kOvV47BkyDB/wSHsSnoqoypblx7CJTNsd2G4m+Zilv6Cxm0i6BbPF1LUbF7Ubi6UOqoLy4Nw24sMoIEhoiJq/CyeFxamP4lyeYzCwRxnnhQ8Ld1gkTqIy8mI59iLCj9R+oTXXwrCbi0fzTCetvDJDIpr8bQ3gCR0QkSXY= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Sep 24, 2026 at 03:48:24PM +0100, Lorenzo Stoakes (ARM) wrote: > 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. > > Fixes: 93c0c8dc87f6 ("mm/rmap: use anon pgoff to track MAP_PRIVATE file-backed anon folios") > Reported-by: syzbot+c181d3198e98f8aef8b9@syzkaller.appspotmail.com > Closes: https://lore.kernel.org/linux-mm/6ab4ae75.80e1c6cc.1e8e5f.000d.GAE@google.com/ > Signed-off-by: Lorenzo Stoakes (ARM) > --- > 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); What's the odds that something "big" actually relies on this? I'm all for the change, but be prepared in about 5 years for people to start complaining :) Want me to take it through my tree? thanks, greg k-h