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Lorenzo Stoakes 6a8b539c3d mm: drop the assumption that VM_SHARED always implies writable 
[ Upstream commit e8e17ee90e ]

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-foundation.org/
[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/d978aefefa83ec42d18dfa964ad180dbcde34795.1697116581.git.lstoakes@gmail.com
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>
2025-08-28 16:21:36 +02:00
arch x86/fpu: Delay instruction pointer fixup until after warning 2025-08-28 16:21:34 +02:00
block
certs
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Documentation
drivers media: venus: hfi: explicitly release IRQ during teardown 2025-08-28 16:21:35 +02:00
fs btrfs: populate otime when logging an inode item 2025-08-28 16:21:35 +02:00
include mm: drop the assumption that VM_SHARED always implies writable 2025-08-28 16:21:36 +02:00
init
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kernel mm: drop the assumption that VM_SHARED always implies writable 2025-08-28 16:21:36 +02:00
lib
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mm mm: drop the assumption that VM_SHARED always implies writable 2025-08-28 16:21:36 +02:00
net codel: remove sch->q.qlen check before qdisc_tree_reduce_backlog() 2025-08-28 16:21:36 +02:00
samples
scripts kbuild: Add KBUILD_CPPFLAGS to as-option invocation 2025-08-28 16:21:33 +02:00
security
sound ALSA: scarlett2: Add retry on -EPROTO from scarlett2_usb_tx() 2025-08-28 16:21:34 +02:00
tools ktest.pl: Prevent recursion of default variable options 2025-08-28 16:21:26 +02:00
usr
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Makefile kbuild: add $(CLANG_FLAGS) to KBUILD_CPPFLAGS 2025-08-28 16:21:33 +02:00
README

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
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In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.