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Fix segfault or stack overflow caused by large derivation fields

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This removes a dynamic stack allocation, making the derivation
unparsing logic robust against overflows when large strings are
added to a derivation.
Overflow behavior depends on the platform and stack configuration.

For instance, x86_64-linux/glibc behaves as (somewhat) expected:

$ (ulimit -s 20000; nix-instantiate tests/lang/eval-okay-big-derivation-attr.nix)
error: stack overflow (possible infinite recursion)

$ (ulimit -s 40000; nix-instantiate tests/lang/eval-okay-big-derivation-attr.nix)
error: expression does not evaluate to a derivation (or a set or list of those)

However, on aarch64-darwin:

$ nix-instantiate big-attr.nix                                                                                                                                                                                                                                                       ~
zsh: segmentation fault  nix-instantiate big-attr.nix

This indicates a slight flaw in the single stack protection page
approach that is not encountered with normal stack frames.
This commit is contained in:
Robert Hensing 2022-01-06 01:20:12 +01:00
parent bc443511eb
commit d038a67bd3
3 changed files with 28 additions and 1 deletions
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10
src/libstore/derivations.cc
View file

@ -272,7 +272,15 @@ Derivation parseDerivation(const Store & store, std::string && s, std::string_vi
static void printString(string & res, std::string_view s) static void printString(string & res, std::string_view s)
{ {
char buf[s.size() * 2 + 2]; char * buf;
size_t bufSize = s.size() * 2 + 2;
std::unique_ptr<char[]> dynBuf;
if (bufSize < 0x10000) {
buf = (char *)alloca(bufSize);
} else {
dynBuf = decltype(dynBuf)(new char[bufSize]);
buf = dynBuf.get();
}
char * p = buf; char * p = buf;
*p++ = '"'; *p++ = '"';
for (auto c : s) for (auto c : s)

13
tests/big-derivation-attr.nix Normal file
View file

@ -0,0 +1,13 @@
let
sixteenBytes = "0123456789abcdef";
times16 = s: builtins.concatStringsSep "" [s s s s s s s s s s s s s s s s];
exp = n: x: if n == 1 then x else times16 (exp (n - 1) x);
sixteenMegabyte = exp 6 sixteenBytes;
in
assert builtins.stringLength sixteenMegabyte == 16777216;
derivation {
name = "big-derivation-attr";
builder = "/x";
system = "y";
bigAttr = sixteenMegabyte;
}

6
tests/simple.sh
View file

@ -25,3 +25,9 @@ if test "$outPath" != "/foo/lfy1s6ca46rm5r6w4gg9hc0axiakjcnm-dependencies.drv";
echo "hashDerivationModulo appears broken, got $outPath" echo "hashDerivationModulo appears broken, got $outPath"
exit 1 exit 1
fi fi
outPath="$(NIX_REMOTE=local?store=/foo\&real=$TEST_ROOT/real-store nix-instantiate --readonly-mode big-derivation-attr.nix)"
if test "$outPath" != "/foo/xxiwa5zlaajv6xdjynf9yym9g319d6mn-big-derivation-attr.drv"; then
echo "big-derivation-attr.nix hash appears broken, got $outPath. Memory corruption in large drv attr?"
exit 1
fi