Hi everyone, I have 2 questions (see comments marked with "Question:")
that I am hoping to get some input on. Any feedback in general you can offer
is greatly appreciated. Most importantly, I would like to be sure that this
is a valid way to implement such a seal. This is my first kernel modification
and I haven't been following the mailing list for very long (for the record
in case there is a dumb mistake in here) I don't know any kernel devs and
figured this would be the most appropriate place to find some useful feedback.
This seal is similar to F_SEAL_WRITE, but will allow the task that created the
memfd to continue writing and retain a single shared writable mapping. Needed for
one-way communication between processes, authenticated at the task level.
Currently the only way to accomplish this is by constantly creating, filling,
sealing write, then sending memfd. Also, a different name suggestion is welcome.
Signed-off-by: Michael R. Tirado <mtirado418@xxxxxxxxx>
---
include/linux/shmem_fs.h | 1 +
include/uapi/linux/fcntl.h | 1 +
kernel/fork.c | 1 +
mm/shmem.c | 77 +++++++++++++++++++--
tools/testing/selftests/memfd/memfd_test.c | 107 +++++++++++++++++++++++++++++
5 files changed, 182 insertions(+), 5 deletions(-)
diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 50777b5..ee25ab3 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -12,6 +12,7 @@
struct shmem_inode_info {
spinlock_t lock;
+ void *creator; /* for authentication only */
unsigned int seals; /* shmem seals */
unsigned long flags;
unsigned long alloced; /* data pages alloced to file */
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index beed138..f339f22 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -40,6 +40,7 @@
#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_WRITE_NONCREATOR 0x0010 /* prevent writes if not creator task */
/* (1U << 31) is reserved for signed error codes */
/*
diff --git a/kernel/fork.c b/kernel/fork.c
index cf65139..f1a35d0 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -434,6 +434,7 @@ static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
if (tmp->vm_flags & VM_DENYWRITE)
atomic_dec(&inode->i_writecount);
i_mmap_lock_write(mapping);
+ /*Question: should this be atomic_inc_unless_negative, or is this negligible since it should never be reached?*/
if (tmp->vm_flags & VM_SHARED)
atomic_inc(&mapping->i_mmap_writable);
flush_dcache_mmap_lock(mapping);
diff --git a/mm/shmem.c b/mm/shmem.c
index cf2d0ca..1e35bc2 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1481,9 +1481,12 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
pgoff_t index = pos >> PAGE_CACHE_SHIFT;
/* i_mutex is held by caller */
- if (unlikely(info->seals)) {
- if (info->seals & F_SEAL_WRITE)
+ if (info->seals) {
+ if (info->seals & F_SEAL_WRITE_NONCREATOR && info->creator == current)
+ goto skip_write_seal;
+ if (info->seals & (F_SEAL_WRITE | F_SEAL_WRITE_NONCREATOR))
return -EPERM;
+skip_write_seal:
if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
return -EPERM;
}
@@ -1938,10 +1941,52 @@ continue_resched:
return error;
}
+/* returns 0 if ok, error if seal cannot be applied */
+static int shmem_seal_noncreator(struct file *file, unsigned int seals,
+ struct shmem_inode_info *info)
+{
+ struct vm_area_struct *vma = NULL;
+ struct vm_area_struct *curvma;
+ int c = 0;
+
+ if (seals & F_SEAL_WRITE || info->seals & F_SEAL_WRITE)
+ return -EPERM; /* these two seals cannot coexist */
+
+ if (atomic_read(&file->f_mapping->i_mmap_writable) == 0
+ || info->seals & F_SEAL_WRITE_NONCREATOR)
+ return 0;
+
+ if (atomic_read(&file->f_mapping->i_mmap_writable) > 1
+ || current != info->creator)
+ return -EPERM;
+
+ /*
+ * search current task vma's for the file
+ * ensure that only one writable shared mapping exists
+ */
+ for (curvma = current->mm->mmap; curvma; curvma = curvma->vm_next) {
+ if (curvma->vm_file == file) {
+ if (curvma->vm_flags & (VM_WRITE | VM_SHARED)) {
+ if (++c > 1)
+ return -EPERM;
+ vma = curvma;
+ }
+ }
+ }
+ if (vma == NULL)
+ return -EPERM;
+
+ vma->vm_flags |= VM_DONTCOPY | VM_DENYWRITE;
+ mapping_unmap_writable(file->f_mapping);
+ return mapping_deny_writable(file->f_mapping);
+}
+
+
#define F_ALL_SEALS (F_SEAL_SEAL | \
F_SEAL_SHRINK | \
F_SEAL_GROW | \
- F_SEAL_WRITE)
+ F_SEAL_WRITE | \
+ F_SEAL_WRITE_NONCREATOR)
int shmem_add_seals(struct file *file, unsigned int seals)
{
@@ -1965,6 +2010,9 @@ int shmem_add_seals(struct file *file, unsigned int seals)
* SEAL_SHRINK: Prevent the file from shrinking
* SEAL_GROW: Prevent the file from growing
* SEAL_WRITE: Prevent write access to the file
+ * SEAL_WRITE_NONCREATOR: same effect as SEAL_WRITE, except the task
+ * that created the file is allowed to write, and
+ * retain a single writable shared mapping.
*
* As we don't require any trust relationship between two parties, we
* must prevent seals from being removed. Therefore, sealing a file
@@ -1993,7 +2041,16 @@ int shmem_add_seals(struct file *file, unsigned int seals)
goto unlock;
}
+ if (seals & F_SEAL_WRITE_NONCREATOR) {
+ error = shmem_seal_noncreator(file, seals, info);
+ if (error)
+ goto unlock;
+ }
if ((seals & F_SEAL_WRITE) && !(info->seals & F_SEAL_WRITE)) {
+ if (info->seals & F_SEAL_WRITE_NONCREATOR) {
+ error = -EPERM;
+ goto unlock;
+ }
error = mapping_deny_writable(file->f_mapping);
if (error)
goto unlock;
@@ -2068,11 +2125,19 @@ 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_NONCREATOR) {
+ if(current == info->creator)
+ goto skip_write_seal;
+ else {
+ error = -EPERM;
+ goto out;
+ }
+ }
if (info->seals & F_SEAL_WRITE) {
error = -EPERM;
goto out;
}
-
+skip_write_seal:
shmem_falloc.waitq = &shmem_falloc_waitq;
shmem_falloc.start = unmap_start >> PAGE_SHIFT;
shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
@@ -2960,8 +3025,10 @@ SYSCALL_DEFINE2(memfd_create,
info = SHMEM_I(file_inode(file));
file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
file->f_flags |= O_RDWR | O_LARGEFILE;
- if (flags & MFD_ALLOW_SEALING)
+ if (flags & MFD_ALLOW_SEALING) {
info->seals &= ~F_SEAL_SEAL;
+ info->creator = current;
+ }/* Question: do we not want a clear info->seals? why the &= ? */
fd_install(fd, file);
kfree(name);
diff --git a/tools/testing/selftests/memfd/memfd_test.c b/tools/testing/selftests/memfd/memfd_test.c
index 0b9eafb..bc1f829 100644
--- a/tools/testing/selftests/memfd/memfd_test.c
+++ b/tools/testing/selftests/memfd/memfd_test.c
@@ -321,6 +321,18 @@ static void mfd_assert_write(int fd)
}
}
+static void mfd_assert_write_nommap(int fd)
+{
+ ssize_t l;
+
+ /* verify write() succeeds */
+ l = write(fd, "\0\0\0\0", 4);
+ if (l != 4) {
+ printf("write() failed: %m\n");
+ abort();
+ }
+}
+
static void mfd_fail_write(int fd)
{
ssize_t l;
@@ -652,6 +664,99 @@ static void test_seal_write(void)
close(fd);
}
+
+/*
+ * Test SEAL_WRITE_NONCREATOR
+ * Test whether SEAL_WRITE_NONCREATOR prevents modifications for all processes
+ * except for the one that created the memfd, and also closes mapping on fork.
+ */
+static void test_seal_write_noncreator()
+{
+ int fd;
+ void *p, *p2, *privmap, *privmap2;
+ pid_t pid;
+ int status;
+
+ fd = mfd_assert_new("kern_memfd_seal_write_noncreator",
+ MFD_DEF_SIZE,
+ MFD_CLOEXEC | MFD_ALLOW_SEALING);
+
+ /* create 2 shared|writes, and one private|read */
+ mfd_assert_has_seals(fd, 0);
+ p = mfd_assert_mmap_shared(fd);
+ p2 = mfd_assert_mmap_shared(fd);
+ privmap = mfd_assert_mmap_private(fd);
+
+ /* verify that seal fails if multiple shared write mappings present*/
+ mfd_fail_add_seals(fd, F_SEAL_WRITE_NONCREATOR);
+ munmap(p2, MFD_DEF_SIZE); /*unmap so theres only 1 shared|write*/
+
+ /* F_SEAL_WRITE_NONCREATOR and F_SEAL_WRITE cannot coexist */
+ mfd_assert_add_seals(fd, F_SEAL_WRITE_NONCREATOR);
+ mfd_assert_has_seals(fd, F_SEAL_WRITE_NONCREATOR);
+ mfd_fail_add_seals(fd, F_SEAL_WRITE);
+
+ /* private mappings with read|write end up having vma with
+ * VM_SHARED set, which this seal checks and will allow only one
+ * to exist. If more than one VM_SHARED exists, the seal fails.
+ * so any private mappings with PROT_WRITE need to be created after
+ * F_SEAL_WRITE_NONCREATOR has been applied.
+ */
+ privmap2 = mmap(NULL, MFD_DEF_SIZE,
+ PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
+ if (privmap2 == MAP_FAILED)
+ abort();
+
+ /* verify that no further shared|write mappings can be made. */
+ p2 = mmap(NULL, MFD_DEF_SIZE,
+ PROT_READ | PROT_WRITE,
+ MAP_SHARED,
+ fd, 0);
+ if (p2 != MAP_FAILED)
+ abort();
+
+ mfd_assert_write_nommap(fd);
+ mfd_assert_read(fd);
+ mfd_assert_shrink(fd);
+ mfd_assert_grow(fd);
+ mfd_assert_grow_write(fd);
+ memset(p, 'A', MFD_DEF_SIZE);
+ memset(privmap2, 'B', MFD_DEF_SIZE);
+
+ /* check authentication */
+ pid = fork();
+ if (pid == 0) /*this new process is not creator, writes should fail*/
+ {
+ mfd_fail_write(fd);
+ mfd_fail_grow_write(fd);
+ mfd_assert_read(fd);
+ if (*(char *)privmap != 'A' || *(char *)privmap2 != 'B')
+ exit(-1); /* just double checking */
+ memset(privmap2, 'Y', MFD_DEF_SIZE);
+ printf("|----: expecting segfault in forked process...\n");
+ memset(p, 'X', MFD_DEF_SIZE);
+ printf("|----: did not crash :(\n");
+ close(fd);
+ exit(-1);
+ }
+
+ /* abort if other process did not crash */
+ pid = waitpid(pid, &status, 0);
+ if (WIFEXITED(status))
+ abort();
+
+ /*tinfoil level error checking */
+ if (*(char *)privmap != 'A'
+ || *(char *)privmap2 != 'B'
+ || *(char *)p != 'A')
+ abort();
+
+ munmap(p, MFD_DEF_SIZE);
+ munmap(privmap, MFD_DEF_SIZE);
+ munmap(privmap2, MFD_DEF_SIZE);
+ close(fd);
+}
+
/*
* Test SEAL_SHRINK
* Test whether SEAL_SHRINK actually prevents shrinking
@@ -882,6 +987,8 @@ int main(int argc, char **argv)
test_seal_grow();
printf("memfd: SEAL-RESIZE\n");
test_seal_resize();
+ printf("memfd: SEAL-WRITE-NONCREATOR\n");
+ test_seal_write_noncreator();
printf("memfd: SHARE-DUP\n");
test_share_dup();
--
1.8.4
--
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