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gpg-interface.c: detect and reject multiple signatures on commits

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GnuPG supports creating signatures consisting of multiple signature
packets.  If such a signature is verified, it outputs all the status
messages for each signature separately.  However, git currently does not
account for such scenario and gets terribly confused over getting
multiple *SIG statuses.

For example, if a malicious party alters a signed commit and appends
a new untrusted signature, git is going to ignore the original bad
signature and report untrusted commit instead.  However, %GK and %GS
format strings may still expand to the data corresponding
to the original signature, potentially tricking the scripts into
trusting the malicious commit.

Given that the use of multiple signatures is quite rare, git does not
support creating them without jumping through a few hoops, and finally
supporting them properly would require extensive API improvement, it
seems reasonable to just reject them at the moment.

Signed-off-by: Michał Górny <mgorny@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Michał Górny
2018-10-20 21:30:20 +02:00
committed by Junio C Hamano
parent cae598d998
commit da6cf1b336
2 changed files with 85 additions and 27 deletions
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26
t/t7510-signed-commit.sh
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@ -234,4 +234,30 @@ test_expect_success GPG 'check config gpg.format values' '
test_must_fail git commit -S --amend -m "fail"
'
test_expect_success GPG 'detect fudged commit with double signature' '
sed -e "/gpgsig/,/END PGP/d" forged1 >double-base &&
sed -n -e "/gpgsig/,/END PGP/p" forged1 | \
sed -e "s/^gpgsig//;s/^ //" | gpg --dearmor >double-sig1.sig &&
gpg -o double-sig2.sig -u 29472784 --detach-sign double-base &&
cat double-sig1.sig double-sig2.sig | gpg --enarmor >double-combined.asc &&
sed -e "s/^\(-.*\)ARMORED FILE/\1SIGNATURE/;1s/^/gpgsig /;2,\$s/^/ /" \
double-combined.asc > double-gpgsig &&
sed -e "/committer/r double-gpgsig" double-base >double-commit &&
git hash-object -w -t commit double-commit >double-commit.commit &&
test_must_fail git verify-commit $(cat double-commit.commit) &&
git show --pretty=short --show-signature $(cat double-commit.commit) >double-actual &&
grep "BAD signature from" double-actual &&
grep "Good signature from" double-actual
'
test_expect_success GPG 'show double signature with custom format' '
cat >expect <<-\EOF &&
E
EOF
git log -1 --format="%G?%n%GK%n%GS" $(cat double-commit.commit) >actual &&
test_cmp expect actual
'
test_done