iio: event: Fix event FIFO reset race

Overview

In the Linux kernel, the following vulnerability has been resolved:

iio: event: Fix event FIFO reset race

iio_event_getfd() creates the event file descriptor with anon_inode_getfd(), which allocates a new fd, creates the anonymous file and installs it in the process fd table before returning to the caller.

The IIO code resets the event FIFO after anon_inode_getfd() has returned, but before IIO_GET_EVENT_FD_IOCTL has copied the fd number to userspace. But since fd tables are shared between threads, another thread can guess the newly allocated fd number and issue a read() on it as soon as the fd has been installed.

This means the kfifo_to_user() in iio_event_chrdev_read() can run in parallel with the kfifo_reset_out() in iio_event_getfd().

The kfifo documentation says that kfifo_reset_out() is only safe when it is called from the reader thread and there is only one concurrent reader. Otherwise it is dangerous and must be handled in the same way as kfifo_reset().

If that happens, kfifo_to_user() can advance the FIFO out index based on state from before the reset, after the reset has already moved the out index to the current in index. That can leave the FIFO with an out index past the in index. A later read() can then see an underflowed FIFO length and copy more data than the event FIFO buffer contains. This can result in an out-of-bounds read and leak adjacent kernel memory to userspace.

Move the FIFO reset before anon_inode_getfd(). At that point the event fd is marked busy, but the new fd has not been installed yet, so userspace cannot access it while the FIFO is reset.

Affected

• Linux >= b91accafbb1031b80d22ad83576877ff2f8b4774 < 9dc84ba4be5bbeb29ee49efe6cea2cb32c461424
• Linux >= b91accafbb1031b80d22ad83576877ff2f8b4774 < d16a702ca7d29c0b7a9b509339d1b044a1cadb32
• Linux >= b91accafbb1031b80d22ad83576877ff2f8b4774 < a13ef1adbc62085b21b546b07b0be7e2fbf52150
• Linux >= b91accafbb1031b80d22ad83576877ff2f8b4774 < 0d4a646d7f87ea3625fafe387043fddc6a2f5e7f
• Linux >= b91accafbb1031b80d22ad83576877ff2f8b4774 < 72c6aa8e0d74eab91b8694cde97dec088c248fee
• Linux >= b91accafbb1031b80d22ad83576877ff2f8b4774 < 9edefd4c56bee3fe331e0355d1f10a533134999d
• Linux >= b91accafbb1031b80d22ad83576877ff2f8b4774 < f187dc5a4c4846ffa07d9bda6e760837ed005574
• Linux >= b91accafbb1031b80d22ad83576877ff2f8b4774 < af791d295737ea6b6ff2c8d8488462a49c14af01
• Linux 3.15

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.