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Subject[PATCH v2] acpi: fix a potential inconsistency caused by double-fetch
Date
"user_buf->length" is in user space, and copied in twice. The second
copy is after it passes the security check. If a user program races to
change user_buf->length in user space, the data fetched in the second
copy may invalidate the security check. The fix avoids the double-fetch
issue by using the value passing the security check.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
---
drivers/acpi/custom_method.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
index 4451877f83b6..f10ee0519033 100644
--- a/drivers/acpi/custom_method.c
+++ b/drivers/acpi/custom_method.c
@@ -26,17 +26,16 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
static u32 max_size;
static u32 uncopied_bytes;

- struct acpi_table_header table;
acpi_status status;

if (!(*ppos)) {
/* parse the table header to get the table length */
if (count <= sizeof(struct acpi_table_header))
return -EINVAL;
- if (copy_from_user(&table, user_buf,
- sizeof(struct acpi_table_header)))
+ if (get_user(max_size,
+ &((struct acpi_table_header *)user_buf)->length))
return -EFAULT;
- uncopied_bytes = max_size = table.length;
+ uncopied_bytes = max_size;
buf = kzalloc(max_size, GFP_KERNEL);
if (!buf)
return -ENOMEM;
@@ -57,6 +56,9 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
return -EFAULT;
}

+ /* Ensure table length is not changed in the second copy */
+ ((struct acpi_table_header *)(buf + (*ppos)))->length = max_size;
+
uncopied_bytes -= count;
*ppos += count;

--
2.17.1
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