[PATCH] linear-assignment: fix potential out of bounds memory access (was: Re: Git 2.19 Segmentation fault 11 on macOS)

[Thomas Gummerer <t.gummerer@gmail.com>]

On 09/11, Thomas Gummerer wrote:
> On 09/11, Thomas Gummerer wrote:
> > I think you're on the right track here.  I can not test this on Mac
> > OS, but on Linux, the following fails when running the test under
> > valgrind:
> > 
> >     diff --git a/t/t3206-range-diff.sh b/t/t3206-range-diff.sh
> >     index 2237c7f4af..a8b0ef8c1d 100755
> >     --- a/t/t3206-range-diff.sh
> >     +++ b/t/t3206-range-diff.sh
> >     @@ -142,4 +142,9 @@ test_expect_success 'changed message' '
> >             test_cmp expected actual
> >      '
> >      
> >     +test_expect_success 'amend and check' '
> >     +       git commit --amend -m "new message" &&
> >     +       git range-diff master HEAD@{1} HEAD
> >     +'
> >     +
> >      test_done
> > 
> > valgrind gives me the following:
> > 
> > ==18232== Invalid read of size 4
> > ==18232==    at 0x34D7B5: compute_assignment (linear-assignment.c:54)
> > ==18232==    by 0x2A4253: get_correspondences (range-diff.c:245)
> > ==18232==    by 0x2A4BFB: show_range_diff (range-diff.c:427)
> > ==18232==    by 0x19D453: cmd_range_diff (range-diff.c:108)
> > ==18232==    by 0x122698: run_builtin (git.c:418)
> > ==18232==    by 0x1229D8: handle_builtin (git.c:637)
> > ==18232==    by 0x122BCC: run_argv (git.c:689)
> > ==18232==    by 0x122D90: cmd_main (git.c:766)
> > ==18232==    by 0x1D55A3: main (common-main.c:45)
> > ==18232==  Address 0x4f4d844 is 0 bytes after a block of size 4 alloc'd
> > ==18232==    at 0x483777F: malloc (vg_replace_malloc.c:299)
> > ==18232==    by 0x3381B0: do_xmalloc (wrapper.c:60)
> > ==18232==    by 0x338283: xmalloc (wrapper.c:87)
> > ==18232==    by 0x2A3F8C: get_correspondences (range-diff.c:207)
> > ==18232==    by 0x2A4BFB: show_range_diff (range-diff.c:427)
> > ==18232==    by 0x19D453: cmd_range_diff (range-diff.c:108)
> > ==18232==    by 0x122698: run_builtin (git.c:418)
> > ==18232==    by 0x1229D8: handle_builtin (git.c:637)
> > ==18232==    by 0x122BCC: run_argv (git.c:689)
> > ==18232==    by 0x122D90: cmd_main (git.c:766)
> > ==18232==    by 0x1D55A3: main (common-main.c:45)
> > ==18232== 
> > 
> > I'm looking into why that fails.  Also adding Dscho to Cc here as the
> > author of this code.
> 
> The diff below seems to fix it.  Not submitting this as a proper
> patch [...]

I found the time to actually have a look at the paper, so here's a
proper patch:

I'm still not entirely sure what the initial code tried to do here,
but I think staying as close as possible to the original is probably
our best option here, also for future readers of this code.

--- >8 ---

Subject: [PATCH] linear-assignment: fix potential out of bounds memory access

Currently the 'compute_assignment()' function can may read memory out
of bounds, even if used correctly.  Namely this happens when we only
have one column.  In that case we try to calculate the initial
minimum cost using '!j1' as column in the reduction transfer code.
That in turn causes us to try and get the cost from column 1 in the
cost matrix, which does not exist, and thus results in an out of
bounds memory read.

Instead of trying to intialize the minimum cost from another column,
just set it to INT_MAX.  This also matches what the example code in the
original paper for the algorithm [1] does (it initializes the value to
inf, for which INT_MAX is the closest match in C).

Note that the test only fails under valgrind on Linux, but the same
command has been reported to segfault on Mac OS.

Also start from 0 in the loop, which matches what the example code in
the original paper does as well.  Starting from 1 means we'd ignore
the first column during the reduction transfer phase.  Note that in
the original paper the loop does start from 1, but the implementation
is in Pascal, where arrays are 1 indexed.

[1]: Jonker, R., & Volgenant, A. (1987). A shortest augmenting path
     algorithm for dense and sparse linear assignment
     problems. Computing, 38(4), 325–340.

Reported-by: ryenus <ryenus@gmail.com>
Helped-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
---
 linear-assignment.c   | 4 ++--
 t/t3206-range-diff.sh | 5 +++++
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/linear-assignment.c b/linear-assignment.c
index 9b3e56e283..7700b80eeb 100644
--- a/linear-assignment.c
+++ b/linear-assignment.c
@@ -51,8 +51,8 @@ void compute_assignment(int column_count, int row_count, int *cost,
 		else if (j1 < -1)
 			row2column[i] = -2 - j1;
 		else {
-			int min = COST(!j1, i) - v[!j1];
-			for (j = 1; j < column_count; j++)
+			int min = INT_MAX;
+			for (j = 0; j < column_count; j++)
 				if (j != j1 && min > COST(j, i) - v[j])
 					min = COST(j, i) - v[j];
 			v[j1] -= min;
diff --git a/t/t3206-range-diff.sh b/t/t3206-range-diff.sh
index 2237c7f4af..fb4c13a84a 100755
--- a/t/t3206-range-diff.sh
+++ b/t/t3206-range-diff.sh
@@ -142,4 +142,9 @@ test_expect_success 'changed message' '
 	test_cmp expected actual
 '
 
+test_expect_success 'no commits on one side' '
+	git commit --amend -m "new message" &&
+	git range-diff master HEAD@{1} HEAD
+'
+
 test_done
-- 
2.19.0.397.gdd90340f6a