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.
As a reminder:
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: 701fac40384f ("iommu/sva: Assign a PASID to mm on PASID allocation and
free it on mm exit")
Reported-by: Zhangfei Gao <zhangfei....@foxmail.com>
Tested-by: Zhangfei Gao <zhangfei....@foxmail.com>
Suggested-by: Jean-Philippe Brucker <jean-phili...@linaro.org>
Suggested-by: Jacob Pan <jacob.jun....@linux.intel.com>
Reviewed-by: Jean-Philippe Brucker <jean-phili...@linaro.org>
Signed-off-by: Fenghua Yu <fenghua...@intel.com>
---
v2:
- Dave Hansen rewrites the change log.
- Add Tested-by: Zhangfei Gao <zhangfei....@foxmail.com>
- Add Reviewed-by: Jean-Philippe Brucker <jean-phili...@linaro.org>
The original patch was posted and discussed in:
https://lore.kernel.org/lkml/ymdzffx7fn586...@fyu1.sc.intel.com/
kernel/fork.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index 9796897560ab..35a3beff140b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -792,6 +792,7 @@ void __mmdrop(struct mm_struct *mm)
mmu_notifier_subscriptions_destroy(mm);
check_mm(mm);
put_user_ns(mm->user_ns);
+ mm_pasid_drop(mm);
free_mm(mm);
}
EXPORT_SYMBOL_GPL(__mmdrop);
@@ -1190,7 +1191,6 @@ static inline void __mmput(struct mm_struct *mm)
}
if (mm->binfmt)
module_put(mm->binfmt->module);
- mm_pasid_drop(mm);
mmdrop(mm);
}
--
2.32.0
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