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ANDROID: ashmem: Add toggle to ignore requests to deny PROT_READ mappings

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Memfd does not support preventing a file from being mapped with PROT_READ,
as ashmem does. It would be useful to expose a knob to userspace to
change ashmem's behavior to match memfd to see if any issues arise
during tests.

Therefore, expose a tunable that userspace can use to cause ashmem to
ignore requests to deny PROT_READ mappings.

Bug: 111903542
Change-Id: Id4d1770e93a4fd5a6b3be04fd82c67d0eff0200e
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>
This commit is contained in:
Isaac J. Manjarres
2025-02-14 17:54:53 -08:00
parent 9f6b96dfca
commit ef10c0ef7d
1 changed files with 31 additions and 0 deletions
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31
drivers/staging/android/ashmem.c
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@@ -107,6 +107,15 @@ static struct lock_class_key backing_shmem_inode_class;
/* Enable unpinning feature by default to retain compatibility with existing behavior. */
static bool unpinning_enable = true;
/*
* memfd does not allow removing permissions to map a buffer with PROT_READ. This variable
* is exposed as a tunable so that it can be used to make ashmem behave more like memfd for
* test purposes.
*
* It is set to false by default to retain compatibility with the original behavior of the driver.
*/
static bool ignore_unset_prot_read;
static inline unsigned long range_size(struct ashmem_range *range)
{
return range->pgend - range->pgstart + 1;
@@ -549,6 +558,10 @@ static int set_prot_mask(struct ashmem_area *asma, unsigned long prot)
mutex_lock(&ashmem_mutex);
/* Ensure the buffer can only be mapped with PROT_READ iff it has that permission. */
if (ignore_unset_prot_read)
prot |= asma->prot_mask & PROT_READ;
/* the user can only remove, not add, protection bits */
if ((asma->prot_mask & prot) != prot) {
ret = -EINVAL;
@@ -934,6 +947,12 @@ static int unpinning_enable_open(struct inode *inode, struct file *file)
return 0;
}
static int ignore_unset_prot_read_open(struct inode *inode, struct file *file)
{
file->private_data = &ignore_unset_prot_read;
return 0;
}
static ssize_t attr_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
{
bool *attrp = file->private_data;
@@ -993,6 +1012,13 @@ static const struct file_operations unpinning_enable_fops = {
.write = attr_write,
};
static const struct file_operations ignore_unset_prot_read_fops = {
.owner = THIS_MODULE,
.open = ignore_unset_prot_read_open,
.read = attr_read,
.write = attr_write,
};
static struct miscdevice ashmem_miscs[] = {
{
.minor = MISC_DYNAMIC_MINOR,
@@ -1004,6 +1030,11 @@ static struct miscdevice ashmem_miscs[] = {
.name = "ashmem_unpinning_enable",
.fops = &unpinning_enable_fops,
},
{
.minor = MISC_DYNAMIC_MINOR,
.name = "ashmem_ignore_unset_prot_read",
.fops = &ignore_unset_prot_read_fops,
},
};
static int __init ashmem_init(void)