many: implement a poor man's privileges drop, use for auth.json #4990
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This PR conceptually implements support for allowing you to do i/o on a file as a less privileged user than you're currently running. The nice, in-process approach doesn't work, as it needs Go 1.10's semantics around runtime.LockOSThread. This instead uses a helper, `privhelper`, that lives in libexec, that knows how to read, write and remove a file, changing to a certain user ahead of time. We can't just use `su`, because su has strange requirements around running from a terminal, and the test suite barfs on it (in practice it might work acceptably, but it being untestable is a big deal). We'd rather not use `sudo`, as this is for something that might already be running under `sudo` and nesting sudos confuses some things; better stay clear. So this PR implements `sys.Setuid` and `sys.Setgid`, with the same caveats as they had in Go 1.3: it only affects the current thread (so LockOSThread rears its head again). This is the first helper that's meant to be run from `snap`, so `cmd.InternalToolPath` would panic at it. Perhaps a second iteration would rework that so that the current approach of having logic in `dirs` is less weird.
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bboozzoo
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Apr 6, 2018
| // For read/remove we could just call cat and rm, but for write we | ||
| // need something custom as we want the write to be atomic. So this | ||
| // helper is needed for the latter case at least; the other two are | ||
| // done here as well for consistency's sake. Furhtermore using su has |
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s/Furhtermore/Furthermore/
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Left some comments in #4983 as well. |
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Three cheers for @bboozzoo for reminding me about euids! back to 4983. |
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This PR conceptually implements support for allowing you to do i/o on
a file as a less privileged user than you're currently running.
The nice, in-process approach doesn't work, as it needs Go 1.10's
semantics around runtime.LockOSThread.
This instead uses a helper,
privhelper, that lives in libexec, thatknows how to read, write and remove a file, changing to a certain user
ahead of time.
We can't just use
su, because su has strange requirements aroundrunning from a terminal, and the test suite barfs on it (in practice
it might work acceptably, but it being untestable is a big deal).
We'd rather not use
sudo, as this is for something that mightalready be running under
sudoand nesting sudos confuses somethings; better stay clear.
So this PR implements
sys.Setuidandsys.Setgid, with the samecaveats as they had in Go 1.3: it only affects the current thread (so
LockOSThread rears its head again).
This is the first helper that's meant to be run from
snap, socmd.InternalToolPathwould panic at it. Perhaps a second iterationwould rework that so that the current approach of having logic in
dirsis less weird.This replaces #4983 and fixes lp:1761193.