lib/shadow/grp/: agetgroups(): Fix possible buffer overrun on non-Linux systems #1295
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…ux systems Linux seems to at least write one group always from getgroups(2). However, POSIX doesn't guarantee this, and a system might have 0 groups. It is implementation‐defined whether getgroups() also returns the effective group ID in the grouplist array. Considering such a system, the call getgroups(0,NULL) could indeed return 0, and the second call to getgroups might return a higher value, if the group list has grown in between (race condition). If this is the case, we'd return an array of 0 elements (or 1, due to the MALLOC() trick to avoid calling it with 0), with no elements filled, but where ngids has been updated to have a positive value. When the caller of agetgroups() reads the array, they'd overrun the buffer. Fixes: 05322ed (2025-01-24; "lib/shadow/grp/: agetgroups(): Add function") Fixes: de941a7 (2025-01-24; "lib/, src/: Simplify allocation of buffer") Signed-off-by: Alejandro Colomar <alx@kernel.org>
hallyn
approved these changes
Jul 16, 2025
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@karelzak I see the exact same bug in util-linux. I guess that even if you don't pretend util-linux to ever run on a non-Linux system, it would be good to write the code portably, in case someone else tries to port it to other systems. Should I send a PR for fixing util-linux? |
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@alejandro-colomar thanks. I'll fix it in util-linux too. It's better to use robust code. |
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Linux seems to at least write one group always from getgroups(2). However, POSIX doesn't guarantee this, and a system might have 0 groups.
Considering such a system, the call getgroups(0,NULL) could indeed return 0, and the second call to getgroups might return a higher value, if the group list has grown in between (race condition). If this is the case, we'd return an array of 0 elements (or 1, due to the MALLOC() trick to avoid calling it with 0), with no elements filled, but where ngids has been updated to have a positive value. When the caller of agetgroups() reads the array, they'd overrun the buffer.
Fixes: 05322ed (2025-01-24; "lib/shadow/grp/: agetgroups(): Add function")
Fixes: de941a7 (2025-01-24; "lib/, src/: Simplify allocation of buffer")