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author | Eric W. Biederman <ebiederm@xmission.com> | 2012-12-29 03:58:39 +0100 |
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committer | Eric W. Biederman <ebiederm@xmission.com> | 2013-01-27 07:11:41 +0100 |
commit | c61a2810a2161986353705b44d9503e6bb079f4f (patch) | |
tree | 1f949f359d0344c2a47c8d0c597815aceb8091c0 /Documentation/DocBook/regulator.tmpl | |
parent | userns: Allow unprivileged reboot (diff) | |
download | linux-c61a2810a2161986353705b44d9503e6bb079f4f.tar.xz linux-c61a2810a2161986353705b44d9503e6bb079f4f.zip |
userns: Avoid recursion in put_user_ns
When freeing a deeply nested user namespace free_user_ns calls
put_user_ns on it's parent which may in turn call free_user_ns again.
When -fno-optimize-sibling-calls is passed to gcc one stack frame per
user namespace is left on the stack, potentially overflowing the
kernel stack. CONFIG_FRAME_POINTER forces -fno-optimize-sibling-calls
so we can't count on gcc to optimize this code.
Remove struct kref and use a plain atomic_t. Making the code more
flexible and easier to comprehend. Make the loop in free_user_ns
explict to guarantee that the stack does not overflow with
CONFIG_FRAME_POINTER enabled.
I have tested this fix with a simple program that uses unshare to
create a deeply nested user namespace structure and then calls exit.
With 1000 nesteuser namespaces before this change running my test
program causes the kernel to die a horrible death. With 10,000,000
nested user namespaces after this change my test program runs to
completion and causes no harm.
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Pointed-out-by: Vasily Kulikov <segoon@openwall.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Diffstat (limited to 'Documentation/DocBook/regulator.tmpl')
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