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The PASID is being freed too early. It needs to stay around until after
device drivers that might be using it have had a chance to clear it out
of the hardware.
The relevant refcounts are:
mmget() /mmput() refcount the mm's address space
mmgrab()/mmdrop() refcount the mm itself
The PASID is currently tied to the life of the mm's address space and freed
in __mmput(). This makes logical sense because the PASID can't be used
once the address space is gone.
But, this misses an important point: even after the address space is gone,
the PASID will still be programmed into a device. Device drivers might,
for instance, still need to flush operations that are outstanding and need
to use that PASID. They do this at file->release() time.
Device drivers call the IOMMU driver to hold a reference on the mm itself
and drop it at file->release() time. But, the IOMMU driver holds a
reference on the mm itself, not the address space. The address space (and
the PASID) is long gone by the time the driver tries to clean up. This is
effectively a use-after-free bug on the PASID.
To fix this, move the PASID free operation from __mmput() to __mmdrop().
This ensures that the IOMMU driver's existing mmgrab() keeps the PASID
allocated until it drops its mm reference.
Fixes:
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| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.