5.4-stable review patch. If anyone has any objections, please let me know.
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From: Lorenzo Stoakes lstoakes@gmail.com
[ Upstream commit e8e17ee90eaf650c855adb0a3e5e965fd6692ff1 ]
Patch series "permit write-sealed memfd read-only shared mappings", v4.
The man page for fcntl() describing memfd file seals states the following about F_SEAL_WRITE:-
Furthermore, trying to create new shared, writable memory-mappings via mmap(2) will also fail with EPERM.
With emphasis on 'writable'. In turns out in fact that currently the kernel simply disallows all new shared memory mappings for a memfd with F_SEAL_WRITE applied, rendering this documentation inaccurate.
This matters because users are therefore unable to obtain a shared mapping to a memfd after write sealing altogether, which limits their usefulness. This was reported in the discussion thread [1] originating from a bug report [2].
This is a product of both using the struct address_space->i_mmap_writable atomic counter to determine whether writing may be permitted, and the kernel adjusting this counter when any VM_SHARED mapping is performed and more generally implicitly assuming VM_SHARED implies writable.
It seems sensible that we should only update this mapping if VM_MAYWRITE is specified, i.e. whether it is possible that this mapping could at any point be written to.
If we do so then all we need to do to permit write seals to function as documented is to clear VM_MAYWRITE when mapping read-only. It turns out this functionality already exists for F_SEAL_FUTURE_WRITE - we can therefore simply adapt this logic to do the same for F_SEAL_WRITE.
We then hit a chicken and egg situation in mmap_region() where the check for VM_MAYWRITE occurs before we are able to clear this flag. To work around this, perform this check after we invoke call_mmap(), with careful consideration of error paths.
Thanks to Andy Lutomirski for the suggestion!
[1]:https://lore.kernel.org/all/20230324133646.16101dfa666f253c4715d965@linux-fo... [2]:https://bugzilla.kernel.org/show_bug.cgi?id=217238
This patch (of 3):
There is a general assumption that VMAs with the VM_SHARED flag set are writable. If the VM_MAYWRITE flag is not set, then this is simply not the case.
Update those checks which affect the struct address_space->i_mmap_writable field to explicitly test for this by introducing [vma_]is_shared_maywrite() helper functions.
This remains entirely conservative, as the lack of VM_MAYWRITE guarantees that the VMA cannot be written to.
Link: https://lkml.kernel.org/r/cover.1697116581.git.lstoakes@gmail.com Link: https://lkml.kernel.org/r/d978aefefa83ec42d18dfa964ad180dbcde34795.169711658... Signed-off-by: Lorenzo Stoakes lstoakes@gmail.com Suggested-by: Andy Lutomirski luto@kernel.org Reviewed-by: Jan Kara jack@suse.cz Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: Christian Brauner brauner@kernel.org Cc: Hugh Dickins hughd@google.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Mike Kravetz mike.kravetz@oracle.com Cc: Muchun Song muchun.song@linux.dev Signed-off-by: Andrew Morton akpm@linux-foundation.org Cc: stable@vger.kernel.org Signed-off-by: Isaac J. Manjarres isaacmanjarres@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/fs.h | 4 ++-- include/linux/mm.h | 11 +++++++++++ kernel/fork.c | 2 +- mm/filemap.c | 2 +- mm/madvise.c | 2 +- mm/mmap.c | 10 +++++----- 6 files changed, 21 insertions(+), 10 deletions(-)
--- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -430,7 +430,7 @@ int pagecache_write_end(struct file *, s * @host: Owner, either the inode or the block_device. * @i_pages: Cached pages. * @gfp_mask: Memory allocation flags to use for allocating pages. - * @i_mmap_writable: Number of VM_SHARED mappings. + * @i_mmap_writable: Number of VM_SHARED, VM_MAYWRITE mappings. * @nr_thps: Number of THPs in the pagecache (non-shmem only). * @i_mmap: Tree of private and shared mappings. * @i_mmap_rwsem: Protects @i_mmap and @i_mmap_writable. @@ -553,7 +553,7 @@ static inline int mapping_mapped(struct
/* * Might pages of this file have been modified in userspace? - * Note that i_mmap_writable counts all VM_SHARED vmas: do_mmap_pgoff + * Note that i_mmap_writable counts all VM_SHARED, VM_MAYWRITE vmas: do_mmap_pgoff * marks vma as VM_SHARED if it is shared, and the file was opened for * writing i.e. vma may be mprotected writable even if now readonly. * --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -549,6 +549,17 @@ static inline bool vma_is_anonymous(stru return !vma->vm_ops; }
+static inline bool is_shared_maywrite(vm_flags_t vm_flags) +{ + return (vm_flags & (VM_SHARED | VM_MAYWRITE)) == + (VM_SHARED | VM_MAYWRITE); +} + +static inline bool vma_is_shared_maywrite(struct vm_area_struct *vma) +{ + return is_shared_maywrite(vma->vm_flags); +} + #ifdef CONFIG_SHMEM /* * The vma_is_shmem is not inline because it is used only by slow --- a/kernel/fork.c +++ b/kernel/fork.c @@ -566,7 +566,7 @@ static __latent_entropy int dup_mmap(str if (tmp->vm_flags & VM_DENYWRITE) atomic_dec(&inode->i_writecount); i_mmap_lock_write(mapping); - if (tmp->vm_flags & VM_SHARED) + if (vma_is_shared_maywrite(tmp)) atomic_inc(&mapping->i_mmap_writable); flush_dcache_mmap_lock(mapping); /* insert tmp into the share list, just after mpnt */ --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2876,7 +2876,7 @@ int generic_file_mmap(struct file * file */ int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma) { - if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE)) + if (vma_is_shared_maywrite(vma)) return -EINVAL; return generic_file_mmap(file, vma); } --- a/mm/madvise.c +++ b/mm/madvise.c @@ -839,7 +839,7 @@ static long madvise_remove(struct vm_are return -EINVAL; }
- if ((vma->vm_flags & (VM_SHARED|VM_WRITE)) != (VM_SHARED|VM_WRITE)) + if (!vma_is_shared_maywrite(vma)) return -EACCES;
offset = (loff_t)(start - vma->vm_start) --- a/mm/mmap.c +++ b/mm/mmap.c @@ -141,7 +141,7 @@ static void __remove_shared_vm_struct(st { if (vma->vm_flags & VM_DENYWRITE) atomic_inc(&file_inode(file)->i_writecount); - if (vma->vm_flags & VM_SHARED) + if (vma_is_shared_maywrite(vma)) mapping_unmap_writable(mapping);
flush_dcache_mmap_lock(mapping); @@ -619,7 +619,7 @@ static void __vma_link_file(struct vm_ar
if (vma->vm_flags & VM_DENYWRITE) atomic_dec(&file_inode(file)->i_writecount); - if (vma->vm_flags & VM_SHARED) + if (vma_is_shared_maywrite(vma)) atomic_inc(&mapping->i_mmap_writable);
flush_dcache_mmap_lock(mapping); @@ -1785,7 +1785,7 @@ unsigned long mmap_region(struct file *f if (error) goto free_vma; } - if (vm_flags & VM_SHARED) { + if (is_shared_maywrite(vm_flags)) { error = mapping_map_writable(file->f_mapping); if (error) goto allow_write_and_free_vma; @@ -1823,7 +1823,7 @@ unsigned long mmap_region(struct file *f vma_link(mm, vma, prev, rb_link, rb_parent); /* Once vma denies write, undo our temporary denial count */ if (file) { - if (vm_flags & VM_SHARED) + if (is_shared_maywrite(vm_flags)) mapping_unmap_writable(file->f_mapping); if (vm_flags & VM_DENYWRITE) allow_write_access(file); @@ -1864,7 +1864,7 @@ unmap_and_free_vma:
/* Undo any partial mapping done by a device driver. */ unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end); - if (vm_flags & VM_SHARED) + if (is_shared_maywrite(vm_flags)) mapping_unmap_writable(file->f_mapping); allow_write_and_free_vma: if (vm_flags & VM_DENYWRITE)