[-next,1/2] mm/memfd: make F_SEAL_FUTURE_WRITE seal more robust
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Message ID 20181120052137.74317-1-joel@joelfernandes.org
State New
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  • [-next,1/2] mm/memfd: make F_SEAL_FUTURE_WRITE seal more robust
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Commit Message

Joel Fernandes Nov. 20, 2018, 5:21 a.m. UTC
A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
where we don't need to modify core VFS structures to get the same
behavior of the seal. This solves several side-effects pointed out by
Andy [2].

[1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/
[2] https://lore.kernel.org/lkml/69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net/

Suggested-by: Andy Lutomirski <luto@kernel.org>
Fixes: 5e653c2923fd ("mm: Add an F_SEAL_FUTURE_WRITE seal to memfd")
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 fs/hugetlbfs/inode.c |  2 +-
 mm/memfd.c           | 19 -------------------
 mm/shmem.c           | 24 +++++++++++++++++++++---
 3 files changed, 22 insertions(+), 23 deletions(-)

Comments

Andy Lutomirski Nov. 20, 2018, 3:13 p.m. UTC | #1
On Mon, Nov 19, 2018 at 9:21 PM Joel Fernandes (Google)
<joel@joelfernandes.org> wrote:
>
> A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
> where we don't need to modify core VFS structures to get the same
> behavior of the seal. This solves several side-effects pointed out by
> Andy [2].
>
> [1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/
> [2] https://lore.kernel.org/lkml/69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net/
>
> Suggested-by: Andy Lutomirski <luto@kernel.org>
> Fixes: 5e653c2923fd ("mm: Add an F_SEAL_FUTURE_WRITE seal to memfd")

What tree is that commit in?  Can we not just fold this in?
Joel Fernandes Nov. 20, 2018, 6:39 p.m. UTC | #2
On Tue, Nov 20, 2018 at 07:13:17AM -0800, Andy Lutomirski wrote:
> On Mon, Nov 19, 2018 at 9:21 PM Joel Fernandes (Google)
> <joel@joelfernandes.org> wrote:
> >
> > A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
> > where we don't need to modify core VFS structures to get the same
> > behavior of the seal. This solves several side-effects pointed out by
> > Andy [2].
> >
> > [1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/
> > [2] https://lore.kernel.org/lkml/69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net/
> >
> > Suggested-by: Andy Lutomirski <luto@kernel.org>
> > Fixes: 5e653c2923fd ("mm: Add an F_SEAL_FUTURE_WRITE seal to memfd")
> 
> What tree is that commit in?  Can we not just fold this in?

It is in linux-next. Could we keep both commits so we have the history?

thanks,

 - Joel
Stephen Rothwell Nov. 20, 2018, 8:07 p.m. UTC | #3
Hi Joel,

On Tue, 20 Nov 2018 10:39:26 -0800 Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Tue, Nov 20, 2018 at 07:13:17AM -0800, Andy Lutomirski wrote:
> > On Mon, Nov 19, 2018 at 9:21 PM Joel Fernandes (Google)
> > <joel@joelfernandes.org> wrote:  
> > >
> > > A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
> > > where we don't need to modify core VFS structures to get the same
> > > behavior of the seal. This solves several side-effects pointed out by
> > > Andy [2].
> > >
> > > [1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/
> > > [2] https://lore.kernel.org/lkml/69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net/
> > >
> > > Suggested-by: Andy Lutomirski <luto@kernel.org>
> > > Fixes: 5e653c2923fd ("mm: Add an F_SEAL_FUTURE_WRITE seal to memfd")  
> > 
> > What tree is that commit in?  Can we not just fold this in?  
> 
> It is in linux-next. Could we keep both commits so we have the history?

Well, its in Andrew's mmotm, so its up to him.
Andy Lutomirski Nov. 20, 2018, 8:33 p.m. UTC | #4
> On Nov 20, 2018, at 1:07 PM, Stephen Rothwell <sfr@canb.auug.org.au> wrote:
> 
> Hi Joel,
> 
>> On Tue, 20 Nov 2018 10:39:26 -0800 Joel Fernandes <joel@joelfernandes.org> wrote:
>> 
>>> On Tue, Nov 20, 2018 at 07:13:17AM -0800, Andy Lutomirski wrote:
>>> On Mon, Nov 19, 2018 at 9:21 PM Joel Fernandes (Google)
>>> <joel@joelfernandes.org> wrote:  
>>>> 
>>>> A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
>>>> where we don't need to modify core VFS structures to get the same
>>>> behavior of the seal. This solves several side-effects pointed out by
>>>> Andy [2].
>>>> 
>>>> [1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/
>>>> [2] https://lore.kernel.org/lkml/69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net/
>>>> 
>>>> Suggested-by: Andy Lutomirski <luto@kernel.org>
>>>> Fixes: 5e653c2923fd ("mm: Add an F_SEAL_FUTURE_WRITE seal to memfd")  
>>> 
>>> What tree is that commit in?  Can we not just fold this in?  
>> 
>> It is in linux-next. Could we keep both commits so we have the history?
> 
> Well, its in Andrew's mmotm, so its up to him.
> 
> 

Unless mmotm is more magical than I think, the commit hash in your fixed tag is already nonsense. mmotm gets rebased all the time, and is only barely a git tree.
Joel Fernandes Nov. 20, 2018, 8:47 p.m. UTC | #5
On Tue, Nov 20, 2018 at 01:33:18PM -0700, Andy Lutomirski wrote:
> 
> > On Nov 20, 2018, at 1:07 PM, Stephen Rothwell <sfr@canb.auug.org.au> wrote:
> > 
> > Hi Joel,
> > 
> >> On Tue, 20 Nov 2018 10:39:26 -0800 Joel Fernandes <joel@joelfernandes.org> wrote:
> >> 
> >>> On Tue, Nov 20, 2018 at 07:13:17AM -0800, Andy Lutomirski wrote:
> >>> On Mon, Nov 19, 2018 at 9:21 PM Joel Fernandes (Google)
> >>> <joel@joelfernandes.org> wrote:  
> >>>> 
> >>>> A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
> >>>> where we don't need to modify core VFS structures to get the same
> >>>> behavior of the seal. This solves several side-effects pointed out by
> >>>> Andy [2].
> >>>> 
> >>>> [1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/
> >>>> [2] https://lore.kernel.org/lkml/69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net/
> >>>> 
> >>>> Suggested-by: Andy Lutomirski <luto@kernel.org>
> >>>> Fixes: 5e653c2923fd ("mm: Add an F_SEAL_FUTURE_WRITE seal to memfd")  
> >>> 
> >>> What tree is that commit in?  Can we not just fold this in?  
> >> 
> >> It is in linux-next. Could we keep both commits so we have the history?
> > 
> > Well, its in Andrew's mmotm, so its up to him.
> > 
> > 
> 
> Unless mmotm is more magical than I think, the commit hash in your fixed
> tag is already nonsense. mmotm gets rebased all the time, and is only
> barely a git tree.

I wouldn't go so far to call it nonsense. It was a working patch, it just did
things differently. Your help with improving the patch is much appreciated.

I am Ok with whatever Andrew wants to do, if it is better to squash it with
the original, then I can do that and send another patch.

- Joel
Andy Lutomirski Nov. 20, 2018, 9:02 p.m. UTC | #6
> On Nov 20, 2018, at 1:47 PM, Joel Fernandes <joel@joelfernandes.org> wrote:
> 
>> On Tue, Nov 20, 2018 at 01:33:18PM -0700, Andy Lutomirski wrote:
>> 
>>> On Nov 20, 2018, at 1:07 PM, Stephen Rothwell <sfr@canb.auug.org.au> wrote:
>>> 
>>> Hi Joel,
>>> 
>>>>> On Tue, 20 Nov 2018 10:39:26 -0800 Joel Fernandes <joel@joelfernandes.org> wrote:
>>>>> 
>>>>> On Tue, Nov 20, 2018 at 07:13:17AM -0800, Andy Lutomirski wrote:
>>>>> On Mon, Nov 19, 2018 at 9:21 PM Joel Fernandes (Google)
>>>>> <joel@joelfernandes.org> wrote:  
>>>>>> 
>>>>>> A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
>>>>>> where we don't need to modify core VFS structures to get the same
>>>>>> behavior of the seal. This solves several side-effects pointed out by
>>>>>> Andy [2].
>>>>>> 
>>>>>> [1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/
>>>>>> [2] https://lore.kernel.org/lkml/69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net/
>>>>>> 
>>>>>> Suggested-by: Andy Lutomirski <luto@kernel.org>
>>>>>> Fixes: 5e653c2923fd ("mm: Add an F_SEAL_FUTURE_WRITE seal to memfd")  
>>>>> 
>>>>> What tree is that commit in?  Can we not just fold this in?  
>>>> 
>>>> It is in linux-next. Could we keep both commits so we have the history?
>>> 
>>> Well, its in Andrew's mmotm, so its up to him.
>>> 
>>> 
>> 
>> Unless mmotm is more magical than I think, the commit hash in your fixed
>> tag is already nonsense. mmotm gets rebased all the time, and is only
>> barely a git tree.
> 
> I wouldn't go so far to call it nonsense. It was a working patch, it just did
> things differently. Your help with improving the patch is much appreciated.

I’m not saying the patch is nonsense — I’m saying the *hash* may be nonsense. akpm uses a bunch of .patch files and all kinds of crazy scripts, and the mmotm.git tree is not stable at all.

> 
> I am Ok with whatever Andrew wants to do, if it is better to squash it with
> the original, then I can do that and send another patch.
> 
> 

From experience, Andrew will food in fixups on request :)
Joel Fernandes Nov. 20, 2018, 9:13 p.m. UTC | #7
On Tue, Nov 20, 2018 at 02:02:49PM -0700, Andy Lutomirski wrote:
> 
> > On Nov 20, 2018, at 1:47 PM, Joel Fernandes <joel@joelfernandes.org> wrote:
> > 
> >> On Tue, Nov 20, 2018 at 01:33:18PM -0700, Andy Lutomirski wrote:
> >> 
> >>> On Nov 20, 2018, at 1:07 PM, Stephen Rothwell <sfr@canb.auug.org.au> wrote:
> >>> 
> >>> Hi Joel,
> >>> 
> >>>>> On Tue, 20 Nov 2018 10:39:26 -0800 Joel Fernandes <joel@joelfernandes.org> wrote:
> >>>>> 
> >>>>> On Tue, Nov 20, 2018 at 07:13:17AM -0800, Andy Lutomirski wrote:
> >>>>> On Mon, Nov 19, 2018 at 9:21 PM Joel Fernandes (Google)
> >>>>> <joel@joelfernandes.org> wrote:  
> >>>>>> 
> >>>>>> A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
> >>>>>> where we don't need to modify core VFS structures to get the same
> >>>>>> behavior of the seal. This solves several side-effects pointed out by
> >>>>>> Andy [2].
> >>>>>> 
> >>>>>> [1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/
> >>>>>> [2] https://lore.kernel.org/lkml/69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net/
> >>>>>> 
> >>>>>> Suggested-by: Andy Lutomirski <luto@kernel.org>
> >>>>>> Fixes: 5e653c2923fd ("mm: Add an F_SEAL_FUTURE_WRITE seal to memfd")  
> >>>>> 
> >>>>> What tree is that commit in?  Can we not just fold this in?  
> >>>> 
> >>>> It is in linux-next. Could we keep both commits so we have the history?
> >>> 
> >>> Well, its in Andrew's mmotm, so its up to him.
> >>> 
> >>> 
> >> 
> >> Unless mmotm is more magical than I think, the commit hash in your fixed
> >> tag is already nonsense. mmotm gets rebased all the time, and is only
> >> barely a git tree.
> > 
> > I wouldn't go so far to call it nonsense. It was a working patch, it just did
> > things differently. Your help with improving the patch is much appreciated.
> 
> I’m not saying the patch is nonsense — I’m saying the *hash* may be
> nonsense. akpm uses a bunch of .patch files and all kinds of crazy scripts,
> and the mmotm.git tree is not stable at all.
> 

Oh, ok. Sorry for misunderstanding and thanks for clarification. :-)

> > I am Ok with whatever Andrew wants to do, if it is better to squash it with
> > the original, then I can do that and send another patch.
> > 
> > 
> 
> From experience, Andrew will food in fixups on request :)

Andrew, could you squash this patch into the one titled ("mm: Add an
F_SEAL_FUTURE_WRITE seal to memfd")? That one was already picked up by -next
but I imagine you might have a crazy script as Andy pointed out for exactly
these situations. ;-)

thanks,

 - Joel
Andrew Morton Nov. 22, 2018, 2:27 a.m. UTC | #8
On Tue, 20 Nov 2018 13:13:35 -0800 Joel Fernandes <joel@joelfernandes.org> wrote:

> > > I am Ok with whatever Andrew wants to do, if it is better to squash it with
> > > the original, then I can do that and send another patch.
> > > 
> > > 
> > 
> > From experience, Andrew will food in fixups on request :)
> 
> Andrew, could you squash this patch into the one titled ("mm: Add an
> F_SEAL_FUTURE_WRITE seal to memfd")? 

Sure.

I could of course queue them separately but I rarely do so - I don't
think that the intermediate development states are useful in the
infinite-term, and I make them available via additional Link: tags in
the changelog footers anyway.

I think that the magnitude of these patches is such that John Stultz's
Reviewed-by is invalidated, so this series is now in the "unreviewed"
state.

So can we have a re-review please?  For convenience, here's the
folded-together [1/1] patch, as it will go to Linus.


From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Subject: mm: Add an F_SEAL_FUTURE_WRITE seal to memfd

Android uses ashmem for sharing memory regions.  We are looking forward to
migrating all usecases of ashmem to memfd so that we can possibly remove
the ashmem driver in the future from staging while also benefiting from
using memfd and contributing to it.  Note staging drivers are also not ABI
and generally can be removed at anytime.

One of the main usecases Android has is the ability to create a region and
mmap it as writeable, then add protection against making any "future"
writes while keeping the existing already mmap'ed writeable-region active.
This allows us to implement a usecase where receivers of the shared
memory buffer can get a read-only view, while the sender continues to
write to the buffer.  See CursorWindow documentation in Android for more
details:
https://developer.android.com/reference/android/database/CursorWindow

This usecase cannot be implemented with the existing F_SEAL_WRITE seal. 
To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
which prevents any future mmap and write syscalls from succeeding while
keeping the existing mmap active.  The following program shows the seal
working in action:

 #include <stdio.h>
 #include <errno.h>
 #include <sys/mman.h>
 #include <linux/memfd.h>
 #include <linux/fcntl.h>
 #include <asm/unistd.h>
 #include <unistd.h>
 #define F_SEAL_FUTURE_WRITE 0x0010
 #define REGION_SIZE (5 * 1024 * 1024)

int memfd_create_region(const char *name, size_t size)
{
    int ret;
    int fd = syscall(__NR_memfd_create, name, MFD_ALLOW_SEALING);
    if (fd < 0) return fd;
    ret = ftruncate(fd, size);
    if (ret < 0) { close(fd); return ret; }
    return fd;
}

int main() {
    int ret, fd;
    void *addr, *addr2, *addr3, *addr1;
    ret = memfd_create_region("test_region", REGION_SIZE);
    printf("ret=%d\n", ret);
    fd = ret;

    // Create map
    addr = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr == MAP_FAILED)
	    printf("map 0 failed\n");
    else
	    printf("map 0 passed\n");

    if ((ret = write(fd, "test", 4)) != 4)
	    printf("write failed even though no future-write seal "
		   "(ret=%d errno =%d)\n", ret, errno);
    else
	    printf("write passed\n");

    addr1 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr1 == MAP_FAILED)
	    perror("map 1 prot-write failed even though no seal\n");
    else
	    printf("map 1 prot-write passed as expected\n");

    ret = fcntl(fd, F_ADD_SEALS, F_SEAL_FUTURE_WRITE |
				 F_SEAL_GROW |
				 F_SEAL_SHRINK);
    if (ret == -1)
	    printf("fcntl failed, errno: %d\n", errno);
    else
	    printf("future-write seal now active\n");

    if ((ret = write(fd, "test", 4)) != 4)
	    printf("write failed as expected due to future-write seal\n");
    else
	    printf("write passed (unexpected)\n");

    addr2 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr2 == MAP_FAILED)
	    perror("map 2 prot-write failed as expected due to seal\n");
    else
	    printf("map 2 passed\n");

    addr3 = mmap(0, REGION_SIZE, PROT_READ, MAP_SHARED, fd, 0);
    if (addr3 == MAP_FAILED)
	    perror("map 3 failed\n");
    else
	    printf("map 3 prot-read passed as expected\n");
}

The output of running this program is as follows:
ret=3
map 0 passed
write passed
map 1 prot-write passed as expected
future-write seal now active
write failed as expected due to future-write seal
map 2 prot-write failed as expected due to seal
: Permission denied
map 3 prot-read passed as expected

[joel@joelfernandes.org: make F_SEAL_FUTURE_WRITE seal more robust]
  Link: http://lkml.kernel.org/r/20181120052137.74317-1-joel@joelfernandes.org
Link: http://lkml.kernel.org/r/20181108041537.39694-1-joel@joelfernandes.org
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: John Reck <jreck@google.com>
Cc: Todd Kjos <tkjos@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Daniel Colascione <dancol@google.com>
Cc: J. Bruce Fields <bfields@fieldses.org>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Khalid Aziz <khalid.aziz@oracle.com>
Cc: Lei Yang <Lei.Yang@windriver.com>
Cc: Marc-Andr Lureau <marcandre.lureau@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---


--- a/include/uapi/linux/fcntl.h~mm-add-an-f_seal_future_write-seal-to-memfd
+++ a/include/uapi/linux/fcntl.h
@@ -41,6 +41,7 @@
 #define F_SEAL_SHRINK	0x0002	/* prevent file from shrinking */
 #define F_SEAL_GROW	0x0004	/* prevent file from growing */
 #define F_SEAL_WRITE	0x0008	/* prevent writes */
+#define F_SEAL_FUTURE_WRITE	0x0010  /* prevent future writes while mapped */
 /* (1U << 31) is reserved for signed error codes */
 
 /*
--- a/mm/memfd.c~mm-add-an-f_seal_future_write-seal-to-memfd
+++ a/mm/memfd.c
@@ -131,7 +131,8 @@ static unsigned int *memfd_file_seals_pt
 #define F_ALL_SEALS (F_SEAL_SEAL | \
 		     F_SEAL_SHRINK | \
 		     F_SEAL_GROW | \
-		     F_SEAL_WRITE)
+		     F_SEAL_WRITE | \
+		     F_SEAL_FUTURE_WRITE)
 
 static int memfd_add_seals(struct file *file, unsigned int seals)
 {
--- a/fs/hugetlbfs/inode.c~mm-add-an-f_seal_future_write-seal-to-memfd
+++ a/fs/hugetlbfs/inode.c
@@ -530,7 +530,7 @@ static long hugetlbfs_punch_hole(struct
 		inode_lock(inode);
 
 		/* protected by i_mutex */
-		if (info->seals & F_SEAL_WRITE) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
 			inode_unlock(inode);
 			return -EPERM;
 		}
--- a/mm/shmem.c~mm-add-an-f_seal_future_write-seal-to-memfd
+++ a/mm/shmem.c
@@ -2119,6 +2119,23 @@ out_nomem:
 
 static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
+	struct shmem_inode_info *info = SHMEM_I(file_inode(file));
+
+	/*
+	 * New PROT_READ and MAP_SHARED mmaps are not allowed when "future
+	 * write" seal active.
+	 */
+	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE) &&
+	    (info->seals & F_SEAL_FUTURE_WRITE))
+		return -EPERM;
+
+	/*
+	 * Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED read-only
+	 * mapping, take care to not allow mprotect to revert protections.
+	 */
+	if (info->seals & F_SEAL_FUTURE_WRITE)
+		vma->vm_flags &= ~(VM_MAYWRITE);
+
 	file_accessed(file);
 	vma->vm_ops = &shmem_vm_ops;
 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
@@ -2344,8 +2361,9 @@ shmem_write_begin(struct file *file, str
 	pgoff_t index = pos >> PAGE_SHIFT;
 
 	/* i_mutex is held by caller */
-	if (unlikely(info->seals & (F_SEAL_WRITE | F_SEAL_GROW))) {
-		if (info->seals & F_SEAL_WRITE)
+	if (unlikely(info->seals & (F_SEAL_GROW |
+				   F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))
 			return -EPERM;
 		if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
 			return -EPERM;
@@ -2608,7 +2626,7 @@ static long shmem_fallocate(struct file
 		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);
 
 		/* protected by i_mutex */
-		if (info->seals & F_SEAL_WRITE) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
 			error = -EPERM;
 			goto out;
 		}
Andy Lutomirski Nov. 22, 2018, 3:25 a.m. UTC | #9
On Wed, Nov 21, 2018 at 6:27 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Tue, 20 Nov 2018 13:13:35 -0800 Joel Fernandes <joel@joelfernandes.org> wrote:
>
> > > > I am Ok with whatever Andrew wants to do, if it is better to squash it with
> > > > the original, then I can do that and send another patch.
> > > >
> > > >
> > >
> > > From experience, Andrew will food in fixups on request :)
> >
> > Andrew, could you squash this patch into the one titled ("mm: Add an
> > F_SEAL_FUTURE_WRITE seal to memfd")?
>
> Sure.
>
> I could of course queue them separately but I rarely do so - I don't
> think that the intermediate development states are useful in the
> infinite-term, and I make them available via additional Link: tags in
> the changelog footers anyway.
>
> I think that the magnitude of these patches is such that John Stultz's
> Reviewed-by is invalidated, so this series is now in the "unreviewed"
> state.
>
> So can we have a re-review please?  For convenience, here's the
> folded-together [1/1] patch, as it will go to Linus.
>
>
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> Subject: mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
>
> Android uses ashmem for sharing memory regions.  We are looking forward to
> migrating all usecases of ashmem to memfd so that we can possibly remove
> the ashmem driver in the future from staging while also benefiting from
> using memfd and contributing to it.  Note staging drivers are also not ABI
> and generally can be removed at anytime.
>
> One of the main usecases Android has is the ability to create a region and
> mmap it as writeable, then add protection against making any "future"
> writes while keeping the existing already mmap'ed writeable-region active.
> This allows us to implement a usecase where receivers of the shared
> memory buffer can get a read-only view, while the sender continues to
> write to the buffer.  See CursorWindow documentation in Android for more
> details:
> https://developer.android.com/reference/android/database/CursorWindow
>
> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
> which prevents any future mmap and write syscalls from succeeding while
> keeping the existing mmap active.  The following program shows the seal
> working in action:
>
>  #include <stdio.h>
>  #include <errno.h>
>  #include <sys/mman.h>
>  #include <linux/memfd.h>
>  #include <linux/fcntl.h>
>  #include <asm/unistd.h>
>  #include <unistd.h>
>  #define F_SEAL_FUTURE_WRITE 0x0010
>  #define REGION_SIZE (5 * 1024 * 1024)
>
> int memfd_create_region(const char *name, size_t size)
> {
>     int ret;
>     int fd = syscall(__NR_memfd_create, name, MFD_ALLOW_SEALING);
>     if (fd < 0) return fd;
>     ret = ftruncate(fd, size);
>     if (ret < 0) { close(fd); return ret; }
>     return fd;
> }
>
> int main() {
>     int ret, fd;
>     void *addr, *addr2, *addr3, *addr1;
>     ret = memfd_create_region("test_region", REGION_SIZE);
>     printf("ret=%d\n", ret);
>     fd = ret;
>
>     // Create map
>     addr = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
>     if (addr == MAP_FAILED)
>             printf("map 0 failed\n");
>     else
>             printf("map 0 passed\n");
>
>     if ((ret = write(fd, "test", 4)) != 4)
>             printf("write failed even though no future-write seal "
>                    "(ret=%d errno =%d)\n", ret, errno);
>     else
>             printf("write passed\n");
>
>     addr1 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
>     if (addr1 == MAP_FAILED)
>             perror("map 1 prot-write failed even though no seal\n");
>     else
>             printf("map 1 prot-write passed as expected\n");
>
>     ret = fcntl(fd, F_ADD_SEALS, F_SEAL_FUTURE_WRITE |
>                                  F_SEAL_GROW |
>                                  F_SEAL_SHRINK);
>     if (ret == -1)
>             printf("fcntl failed, errno: %d\n", errno);
>     else
>             printf("future-write seal now active\n");
>
>     if ((ret = write(fd, "test", 4)) != 4)
>             printf("write failed as expected due to future-write seal\n");
>     else
>             printf("write passed (unexpected)\n");
>
>     addr2 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
>     if (addr2 == MAP_FAILED)
>             perror("map 2 prot-write failed as expected due to seal\n");
>     else
>             printf("map 2 passed\n");
>
>     addr3 = mmap(0, REGION_SIZE, PROT_READ, MAP_SHARED, fd, 0);
>     if (addr3 == MAP_FAILED)
>             perror("map 3 failed\n");
>     else
>             printf("map 3 prot-read passed as expected\n");
> }
>
> The output of running this program is as follows:
> ret=3
> map 0 passed
> write passed
> map 1 prot-write passed as expected
> future-write seal now active
> write failed as expected due to future-write seal
> map 2 prot-write failed as expected due to seal
> : Permission denied
> map 3 prot-read passed as expected
>
> [joel@joelfernandes.org: make F_SEAL_FUTURE_WRITE seal more robust]
>   Link: http://lkml.kernel.org/r/20181120052137.74317-1-joel@joelfernandes.org
> Link: http://lkml.kernel.org/r/20181108041537.39694-1-joel@joelfernandes.org
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Cc: John Stultz <john.stultz@linaro.org>
> Cc: John Reck <jreck@google.com>
> Cc: Todd Kjos <tkjos@google.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Daniel Colascione <dancol@google.com>
> Cc: J. Bruce Fields <bfields@fieldses.org>
> Cc: Jeff Layton <jlayton@kernel.org>
> Cc: Khalid Aziz <khalid.aziz@oracle.com>
> Cc: Lei Yang <Lei.Yang@windriver.com>
> Cc: Marc-Andr Lureau <marcandre.lureau@redhat.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Valdis Kletnieks <valdis.kletnieks@vt.edu>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Jann Horn <jannh@google.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
>
> --- a/include/uapi/linux/fcntl.h~mm-add-an-f_seal_future_write-seal-to-memfd
> +++ a/include/uapi/linux/fcntl.h
> @@ -41,6 +41,7 @@
>  #define F_SEAL_SHRINK  0x0002  /* prevent file from shrinking */
>  #define F_SEAL_GROW    0x0004  /* prevent file from growing */
>  #define F_SEAL_WRITE   0x0008  /* prevent writes */
> +#define F_SEAL_FUTURE_WRITE    0x0010  /* prevent future writes while mapped */
>  /* (1U << 31) is reserved for signed error codes */
>
>  /*
> --- a/mm/memfd.c~mm-add-an-f_seal_future_write-seal-to-memfd
> +++ a/mm/memfd.c
> @@ -131,7 +131,8 @@ static unsigned int *memfd_file_seals_pt
>  #define F_ALL_SEALS (F_SEAL_SEAL | \
>                      F_SEAL_SHRINK | \
>                      F_SEAL_GROW | \
> -                    F_SEAL_WRITE)
> +                    F_SEAL_WRITE | \
> +                    F_SEAL_FUTURE_WRITE)
>
>  static int memfd_add_seals(struct file *file, unsigned int seals)
>  {
> --- a/fs/hugetlbfs/inode.c~mm-add-an-f_seal_future_write-seal-to-memfd
> +++ a/fs/hugetlbfs/inode.c
> @@ -530,7 +530,7 @@ static long hugetlbfs_punch_hole(struct
>                 inode_lock(inode);
>
>                 /* protected by i_mutex */
> -               if (info->seals & F_SEAL_WRITE) {
> +               if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
>                         inode_unlock(inode);
>                         return -EPERM;
>                 }
> --- a/mm/shmem.c~mm-add-an-f_seal_future_write-seal-to-memfd
> +++ a/mm/shmem.c
> @@ -2119,6 +2119,23 @@ out_nomem:
>
>  static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
>  {
> +       struct shmem_inode_info *info = SHMEM_I(file_inode(file));
> +
> +       /*
> +        * New PROT_READ and MAP_SHARED mmaps are not allowed when "future

PROT_WRITE, perhaps?

> +        * write" seal active.
> +        */
> +       if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE) &&
> +           (info->seals & F_SEAL_FUTURE_WRITE))
> +               return -EPERM;
> +
> +       /*
> +        * Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED read-only
> +        * mapping, take care to not allow mprotect to revert protections.
> +        */
> +       if (info->seals & F_SEAL_FUTURE_WRITE)
> +               vma->vm_flags &= ~(VM_MAYWRITE);
> +

This might all be clearer as:

if (info->seals & F_SEAL_FUTURE_WRITE) {
  if (vma->vm_flags ...)
    return -EPERM;
  vma->vm_flags &= ~VM_MAYWRITE;
}

with appropriate comments inserted.
Joel Fernandes Nov. 22, 2018, 11:09 p.m. UTC | #10
On Wed, Nov 21, 2018 at 07:25:26PM -0800, Andy Lutomirski wrote:
> On Wed, Nov 21, 2018 at 6:27 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Tue, 20 Nov 2018 13:13:35 -0800 Joel Fernandes <joel@joelfernandes.org> wrote:
> >
> > > > > I am Ok with whatever Andrew wants to do, if it is better to squash it with
> > > > > the original, then I can do that and send another patch.
> > > > >
> > > > >
> > > >
> > > > From experience, Andrew will food in fixups on request :)
> > >
> > > Andrew, could you squash this patch into the one titled ("mm: Add an
> > > F_SEAL_FUTURE_WRITE seal to memfd")?
> >
> > Sure.
> >
> > I could of course queue them separately but I rarely do so - I don't
> > think that the intermediate development states are useful in the
> > infinite-term, and I make them available via additional Link: tags in
> > the changelog footers anyway.
> >
> > I think that the magnitude of these patches is such that John Stultz's
> > Reviewed-by is invalidated, so this series is now in the "unreviewed"
> > state.
> >
> > So can we have a re-review please?  For convenience, here's the
> > folded-together [1/1] patch, as it will go to Linus.

Sure, I removed the old tags and also provide an updated patch below inline.

> > From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> > Subject: mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
> >
> > Android uses ashmem for sharing memory regions.  We are looking forward to
> > migrating all usecases of ashmem to memfd so that we can possibly remove
> > the ashmem driver in the future from staging while also benefiting from
> > using memfd and contributing to it.  Note staging drivers are also not ABI
> > and generally can be removed at anytime.
[...]
> > --- a/include/uapi/linux/fcntl.h~mm-add-an-f_seal_future_write-seal-to-memfd
> > +++ a/include/uapi/linux/fcntl.h
> > @@ -41,6 +41,7 @@
> >  #define F_SEAL_SHRINK  0x0002  /* prevent file from shrinking */
> >  #define F_SEAL_GROW    0x0004  /* prevent file from growing */
> >  #define F_SEAL_WRITE   0x0008  /* prevent writes */
> > +#define F_SEAL_FUTURE_WRITE    0x0010  /* prevent future writes while mapped */
> >  /* (1U << 31) is reserved for signed error codes */
> >
> >  /*
> > --- a/mm/memfd.c~mm-add-an-f_seal_future_write-seal-to-memfd
> > +++ a/mm/memfd.c
> > @@ -131,7 +131,8 @@ static unsigned int *memfd_file_seals_pt
> >  #define F_ALL_SEALS (F_SEAL_SEAL | \
> >                      F_SEAL_SHRINK | \
> >                      F_SEAL_GROW | \
> > -                    F_SEAL_WRITE)
> > +                    F_SEAL_WRITE | \
> > +                    F_SEAL_FUTURE_WRITE)
> >
> >  static int memfd_add_seals(struct file *file, unsigned int seals)
> >  {
> > --- a/fs/hugetlbfs/inode.c~mm-add-an-f_seal_future_write-seal-to-memfd
> > +++ a/fs/hugetlbfs/inode.c
> > @@ -530,7 +530,7 @@ static long hugetlbfs_punch_hole(struct
> >                 inode_lock(inode);
> >
> >                 /* protected by i_mutex */
> > -               if (info->seals & F_SEAL_WRITE) {
> > +               if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
> >                         inode_unlock(inode);
> >                         return -EPERM;
> >                 }
> > --- a/mm/shmem.c~mm-add-an-f_seal_future_write-seal-to-memfd
> > +++ a/mm/shmem.c
> > @@ -2119,6 +2119,23 @@ out_nomem:
> >
> >  static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
> >  {
> > +       struct shmem_inode_info *info = SHMEM_I(file_inode(file));
> > +
> > +       /*
> > +        * New PROT_READ and MAP_SHARED mmaps are not allowed when "future
> 
> PROT_WRITE, perhaps?

Yes, fixed.

> > +        * write" seal active.
> > +        */
> > +       if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE) &&
> > +           (info->seals & F_SEAL_FUTURE_WRITE))
> > +               return -EPERM;
> > +
> > +       /*
> > +        * Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED read-only
> > +        * mapping, take care to not allow mprotect to revert protections.
> > +        */
> > +       if (info->seals & F_SEAL_FUTURE_WRITE)
> > +               vma->vm_flags &= ~(VM_MAYWRITE);
> > +
> 
> This might all be clearer as:
> 
> if (info->seals & F_SEAL_FUTURE_WRITE) {
>   if (vma->vm_flags ...)
>     return -EPERM;
>   vma->vm_flags &= ~VM_MAYWRITE;
> }
> 
> with appropriate comments inserted.

Agreed, its simpler. Updated patch is below. I squashed it with all the
earlier ones. Andy, could you provide Acks and/or Reviewed-by tag as well?

---8<-----------------------

From b5a4960e755af67e9f6f9e65db5113e712cf338e Mon Sep 17 00:00:00 2001
From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Date: Sat, 10 Nov 2018 22:21:31 -0800
Subject: [PATCH v4] mm/memfd: Add an F_SEAL_FUTURE_WRITE seal to memfd

Android uses ashmem for sharing memory regions.  We are looking forward to
migrating all usecases of ashmem to memfd so that we can possibly remove
the ashmem driver in the future from staging while also benefiting from
using memfd and contributing to it.  Note staging drivers are also not ABI
and generally can be removed at anytime.

One of the main usecases Android has is the ability to create a region and
mmap it as writeable, then add protection against making any "future"
writes while keeping the existing already mmap'ed writeable-region active.
This allows us to implement a usecase where receivers of the shared
memory buffer can get a read-only view, while the sender continues to
write to the buffer.  See CursorWindow documentation in Android for more
details:
https://developer.android.com/reference/android/database/CursorWindow

This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
which prevents any future mmap and write syscalls from succeeding while
keeping the existing mmap active.

A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
where we don't need to modify core VFS structures to get the same
behavior of the seal. This solves several side-effects pointed by Andy.
self-tests are provided in later patch to verify the expected semantics.

[1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 fs/hugetlbfs/inode.c       |  2 +-
 include/uapi/linux/fcntl.h |  1 +
 mm/memfd.c                 |  3 ++-
 mm/shmem.c                 | 26 +++++++++++++++++++++++---
 4 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 762028994f47..5b54bf893a67 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -558,7 +558,7 @@ static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
 		inode_lock(inode);
 
 		/* protected by i_mutex */
-		if (info->seals & F_SEAL_WRITE) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
 			inode_unlock(inode);
 			return -EPERM;
 		}
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 594b85f7cb86..1d338357df8a 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -41,6 +41,7 @@
 #define F_SEAL_SHRINK	0x0002	/* prevent file from shrinking */
 #define F_SEAL_GROW	0x0004	/* prevent file from growing */
 #define F_SEAL_WRITE	0x0008	/* prevent writes */
+#define F_SEAL_FUTURE_WRITE	0x0010  /* prevent future writes while mapped */
 /* (1U << 31) is reserved for signed error codes */
 
 /*
diff --git a/mm/memfd.c b/mm/memfd.c
index 97264c79d2cd..650e65a46b9c 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -131,7 +131,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file)
 #define F_ALL_SEALS (F_SEAL_SEAL | \
 		     F_SEAL_SHRINK | \
 		     F_SEAL_GROW | \
-		     F_SEAL_WRITE)
+		     F_SEAL_WRITE | \
+		     F_SEAL_FUTURE_WRITE)
 
 static int memfd_add_seals(struct file *file, unsigned int seals)
 {
diff --git a/mm/shmem.c b/mm/shmem.c
index 32eb29bd72c6..f5069e8225cc 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2121,6 +2121,25 @@ int shmem_lock(struct file *file, int lock, struct user_struct *user)
 
 static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
+	struct shmem_inode_info *info = SHMEM_I(file_inode(file));
+
+
+	if (info->seals & F_SEAL_FUTURE_WRITE) {
+		/*
+		 * New PROT_WRITE and MAP_SHARED mmaps are not allowed when
+		 * "future write" seal active.
+		 */
+		if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE))
+			return -EPERM;
+
+		/*
+		 * Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED
+		 * read-only mapping, take care to not allow mprotect to revert
+		 * protections.
+		 */
+		vma->vm_flags &= ~(VM_MAYWRITE);
+	}
+
 	file_accessed(file);
 	vma->vm_ops = &shmem_vm_ops;
 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
@@ -2346,8 +2365,9 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
 	pgoff_t index = pos >> PAGE_SHIFT;
 
 	/* i_mutex is held by caller */
-	if (unlikely(info->seals & (F_SEAL_WRITE | F_SEAL_GROW))) {
-		if (info->seals & F_SEAL_WRITE)
+	if (unlikely(info->seals & (F_SEAL_GROW |
+				   F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))
 			return -EPERM;
 		if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
 			return -EPERM;
@@ -2610,7 +2630,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);
 
 		/* protected by i_mutex */
-		if (info->seals & F_SEAL_WRITE) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
 			error = -EPERM;
 			goto out;
 		}
Andrew Morton Nov. 25, 2018, 12:42 a.m. UTC | #11
On Thu, 22 Nov 2018 15:09:06 -0800 Joel Fernandes <joel@joelfernandes.org> wrote:

> Android uses ashmem for sharing memory regions.  We are looking forward to
> migrating all usecases of ashmem to memfd so that we can possibly remove
> the ashmem driver in the future from staging while also benefiting from
> using memfd and contributing to it.  Note staging drivers are also not ABI
> and generally can be removed at anytime.
> 
> One of the main usecases Android has is the ability to create a region and
> mmap it as writeable, then add protection against making any "future"
> writes while keeping the existing already mmap'ed writeable-region active.
> This allows us to implement a usecase where receivers of the shared
> memory buffer can get a read-only view, while the sender continues to
> write to the buffer.  See CursorWindow documentation in Android for more
> details:
> https://developer.android.com/reference/android/database/CursorWindow
> 
> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
> which prevents any future mmap and write syscalls from succeeding while
> keeping the existing mmap active.
> 
> A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
> where we don't need to modify core VFS structures to get the same
> behavior of the seal. This solves several side-effects pointed by Andy.
> self-tests are provided in later patch to verify the expected semantics.
> 
> [1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/

This changelog doesn't have the nifty test case code which was in
earlier versions?
Matthew Wilcox Nov. 25, 2018, 12:47 a.m. UTC | #12
On Sat, Nov 24, 2018 at 04:42:29PM -0800, Andrew Morton wrote:
> This changelog doesn't have the nifty test case code which was in
> earlier versions?

Why do we put regression tests in the changelogs anyway?  We have
tools/testing/selftests/vm/ already, perhaps they should go there?
Joel Fernandes Nov. 26, 2018, 1:35 p.m. UTC | #13
On Sat, Nov 24, 2018 at 04:47:36PM -0800, Matthew Wilcox wrote:
> On Sat, Nov 24, 2018 at 04:42:29PM -0800, Andrew Morton wrote:
> > This changelog doesn't have the nifty test case code which was in
> > earlier versions?
> 
> Why do we put regression tests in the changelogs anyway?  We have
> tools/testing/selftests/vm/ already, perhaps they should go there?

The reason is I didn't add it was that test case went out of date and the
updated version of the test case went into the selftests in patch 2/2. I
thought that would suffice which covers all the cases. That's why I dropped
it.  Would that be Ok?

The changelog of the previous series had it because the selftest was added
only later.

Let me know, thanks,

 - Joel

Patch
diff mbox series

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 762028994f47..5b54bf893a67 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -558,7 +558,7 @@  static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
 		inode_lock(inode);
 
 		/* protected by i_mutex */
-		if (info->seals & F_SEAL_WRITE) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
 			inode_unlock(inode);
 			return -EPERM;
 		}
diff --git a/mm/memfd.c b/mm/memfd.c
index 63fff5e77114..650e65a46b9c 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -201,25 +201,6 @@  static int memfd_add_seals(struct file *file, unsigned int seals)
 		}
 	}
 
-	if ((seals & F_SEAL_FUTURE_WRITE) &&
-	    !(*file_seals & F_SEAL_FUTURE_WRITE)) {
-		/*
-		 * The FUTURE_WRITE seal also prevents growing and shrinking
-		 * so we need them to be already set, or requested now.
-		 */
-		int test_seals = (seals | *file_seals) &
-				 (F_SEAL_GROW | F_SEAL_SHRINK);
-
-		if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
-			error = -EINVAL;
-			goto unlock;
-		}
-
-		spin_lock(&file->f_lock);
-		file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
-		spin_unlock(&file->f_lock);
-	}
-
 	*file_seals |= seals;
 	error = 0;
 
diff --git a/mm/shmem.c b/mm/shmem.c
index 32eb29bd72c6..cee9878c87f1 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2121,6 +2121,23 @@  int shmem_lock(struct file *file, int lock, struct user_struct *user)
 
 static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
+	struct shmem_inode_info *info = SHMEM_I(file_inode(file));
+
+	/*
+	 * New PROT_READ and MAP_SHARED mmaps are not allowed when "future
+	 * write" seal active.
+	 */
+	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE) &&
+	    (info->seals & F_SEAL_FUTURE_WRITE))
+		return -EPERM;
+
+	/*
+	 * Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED read-only
+	 * mapping, take care to not allow mprotect to revert protections.
+	 */
+	if (info->seals & F_SEAL_FUTURE_WRITE)
+		vma->vm_flags &= ~(VM_MAYWRITE);
+
 	file_accessed(file);
 	vma->vm_ops = &shmem_vm_ops;
 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
@@ -2346,8 +2363,9 @@  shmem_write_begin(struct file *file, struct address_space *mapping,
 	pgoff_t index = pos >> PAGE_SHIFT;
 
 	/* i_mutex is held by caller */
-	if (unlikely(info->seals & (F_SEAL_WRITE | F_SEAL_GROW))) {
-		if (info->seals & F_SEAL_WRITE)
+	if (unlikely(info->seals & (F_SEAL_GROW |
+				   F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))
 			return -EPERM;
 		if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
 			return -EPERM;
@@ -2610,7 +2628,7 @@  static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);
 
 		/* protected by i_mutex */
-		if (info->seals & F_SEAL_WRITE) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
 			error = -EPERM;
 			goto out;
 		}