** Changed in: linux (Ubuntu)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1851677
Title:
s_iflags overlap prevents unprivileged overlayfs mounts
Status in linux package in Ubuntu:
Fix Committed
Status in linux source package in Disco:
In Progress
Status in linux source package in Eoan:
In Progress
Bug description:
SRU Justification
Impact: SB_I_NOSUID was added by a sauce patch, and over time it has
come to occpy the same bit in s_iflags as SB_I_USERNS_VISIBLE without
being noticed. overlayfs will set SB_I_NOSUID when any lower mount is
nosuid. When this happens for a user namespace mount,
mount_too_revealing() will perform additional, unnecessary checks
which may block mounting when it should be allowed.
Fix: Relocate SB_I_NOSUID to the top of s_iflags to remove the
conflict and to make future conflicts less likely.
Test Case: Attached.
Regression Potential: These flags are internal to the kernel, so
moving SB_I_NOSUID is safe. It's possible that the overlapping flags
caused other unintended behaviors, and if so these will also change
and could result in user-visible changes. However, any such behaviors
would also be incorrect.
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As reported by Jann Horn on bug 1850994, unprivileged overlayfs
mounting can fail in disco/eoan due to SB_I_NOSUID and
SB_I_USERNS_VISIBLE. overlayfs sets SB_I_NOSUID in some cases, which
causes mount_too_revealing() to see SB_I_USERNS_VISIBLE as being set.
This leads to performing additional, unnecessary checks which may
fail.
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