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Subject[PATCH] sched: fix a potential double-fetch bug in sched_copy_attr
Date
"uattr->size" is copied in from user space and checked. However, it is
copied in again after the security check. A malicious user may race to
change it. The fix checks if uattr->size is ever changed after the
check.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
---
kernel/sched/core.c | 18 ++++++++++++------
1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6fedf3a98581..0a55bdce9a42 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4447,7 +4447,7 @@ do_sched_setscheduler(pid_t pid, int policy, struct sched_param __user *param)
*/
static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *attr)
{
- u32 size;
+ u32 size, size_cp;
int ret;

if (!access_ok(VERIFY_WRITE, uattr, SCHED_ATTR_SIZE_VER0))
@@ -4460,15 +4460,17 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
if (ret)
return ret;

+ size_cp = size;
+
/* Bail out on silly large: */
if (size > PAGE_SIZE)
goto err_size;

/* ABI compatibility quirk: */
if (!size)
- size = SCHED_ATTR_SIZE_VER0;
+ size_cp = SCHED_ATTR_SIZE_VER0;

- if (size < SCHED_ATTR_SIZE_VER0)
+ else if (size < SCHED_ATTR_SIZE_VER0)
goto err_size;

/*
@@ -4483,7 +4485,7 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
unsigned char val;

addr = (void __user *)uattr + sizeof(*attr);
- end = (void __user *)uattr + size;
+ end = (void __user *)uattr + size_cp;

for (; addr < end; addr++) {
ret = get_user(val, addr);
@@ -4492,13 +4494,17 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
if (val)
goto err_size;
}
- size = sizeof(*attr);
+ size_cp = sizeof(*attr);
}

- ret = copy_from_user(attr, uattr, size);
+ ret = copy_from_user(attr, uattr, size_cp);
if (ret)
return -EFAULT;

+ /* Sanity check if size was changed in user space */
+ if (attr->size != size)
+ return -EINVAL;
+
/*
* XXX: Do we want to be lenient like existing syscalls; or do we want
* to be strict and return an error on out-of-bounds values?
--
2.17.2 (Apple Git-113)
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