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Security: Don't allow builders to change permissions on files they don't own

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It turns out that in multi-user Nix, a builder may be able to do

  ln /etc/shadow $out/foo

Afterwards, canonicalisePathMetaData() will be applied to $out/foo,
causing /etc/shadow's mode to be set to 444 (readable by everybody but
writable by nobody).  That's obviously Very Bad.

Fortunately, this fails in NixOS's default configuration because
/nix/store is a bind mount, so "ln" will fail with "Invalid
cross-device link".  It also fails if hard-link restrictions are
enabled, so a workaround is:

  echo 1 > /proc/sys/fs/protected_hardlinks

The solution is to check that all files in $out are owned by the build
user.  This means that innocuous operations like "ln
${pkgs.foo}/some-file $out/" are now rejected, but that already failed
in chroot builds anyway.
This commit is contained in:
Eelco Dolstra 2013-02-26 02:30:19 +01:00
parent dadf7a5b46
commit 5526a282b5
5 changed files with 17 additions and 20 deletions
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10
src/nix-store/nix-store.cc
View file

@ -238,12 +238,6 @@ static void printTree(const Path & path,
PathSet references;
store->queryReferences(path, references);
#if 0
for (PathSet::iterator i = drv.inputSrcs.begin();
i != drv.inputSrcs.end(); ++i)
cout << format("%1%%2%\n") % (tailPad + treeConn) % *i;
#endif
/* Topologically sort under the relation A < B iff A \in
closure(B). That is, if derivation A is an (possibly indirect)
input of B, then A is printed first. This has the effect of
@ -251,7 +245,7 @@ static void printTree(const Path & path,
Paths sorted = topoSortPaths(*store, references);
reverse(sorted.begin(), sorted.end());
for (Paths::iterator i = sorted.begin(); i != sorted.end(); ++i) {
foreach (Paths::iterator, i, sorted) {
Paths::iterator j = i; ++j;
printTree(*i, tailPad + treeConn,
j == sorted.end() ? tailPad + treeNull : tailPad + treeLine,
@ -521,7 +515,7 @@ static void registerValidity(bool reregister, bool hashGiven, bool canonicalise)
if (!store->isValidPath(info.path) || reregister) {
/* !!! races */
if (canonicalise)
canonicalisePathMetaData(info.path);
canonicalisePathMetaData(info.path, -1);
if (!hashGiven) {
HashResult hash = hashPath(htSHA256, info.path);
info.hash = hash.first;