[PATCH] GDB: Fix out of bounds accesses with limited-length values

Maciej W. Rozycki
2023-02-23T21:14:45Z

Fix accesses to limited-length values in `contents_copy_raw' and
`contents_copy_raw_bitwise' so that they observe the limit of the
original allocation.

Reported by Simon Marchi as a heap-buffer-overflow AddressSanitizer
issue triggered with gdb.ada/limited-length.exp.
---
Hi,

Verified to remove the original issue and not to cause any regressions
with and w/o AddressSanitizer and native `x86_64-linux-gnu'. OK to apply?

Maciej
---
gdb/value.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)

gdb-limited-length-array-value-contents-copy-fix.diff
Index: src/gdb/value.c
===================================================================
--- src.orig/gdb/value.c
+++ src/gdb/value.c
@@ -1168,6 +1168,11 @@ value::contents_copy_raw (struct value *
mean we'd be copying garbage. */
gdb_assert (!dst->m_lazy && !m_lazy);

+ ULONGEST copy_length = length;
+ ULONGEST limit = m_limited_length;
+ if (limit > 0 && src_offset + length > limit)
+ copy_length = src_offset > limit ? 0 : limit - src_offset;
+
/* The overwritten DST range gets unavailability ORed in, not
replaced. Make sure to remember to implement replacing if it
turns out actually necessary. */
@@ -1178,10 +1183,10 @@ value::contents_copy_raw (struct value *
/* Copy the data. */
gdb::array_view<gdb_byte> dst_contents
= dst->contents_all_raw ().slice (dst_offset * unit_size,
- length * unit_size);
+ copy_length * unit_size);
gdb::array_view<const gdb_byte> src_contents
= contents_all_raw ().slice (src_offset * unit_size,
- length * unit_size);
+ copy_length * unit_size);
gdb::copy (src_contents, dst_contents);

/* Copy the meta-data, adjusted. */
@@ -1206,6 +1211,12 @@ value::contents_copy_raw_bitwise (struct
mean we'd be copying garbage. */
gdb_assert (!dst->m_lazy && !m_lazy);

+ ULONGEST copy_bit_length = bit_length;
+ ULONGEST bit_limit = m_limited_length * TARGET_CHAR_BIT;
+ if (bit_limit > 0 && src_bit_offset + bit_length > bit_limit)
+ copy_bit_length = (src_bit_offset > bit_limit ? 0
+ : bit_limit - src_bit_offset);
+
/* The overwritten DST range gets unavailability ORed in, not
replaced. Make sure to remember to implement replacing if it
turns out actually necessary. */
@@ -1220,7 +1231,7 @@ value::contents_copy_raw_bitwise (struct
gdb::array_view<const gdb_byte> src_contents = contents_all_raw ();
copy_bitwise (dst_contents.data (), dst_bit_offset,
src_contents.data (), src_bit_offset,
- bit_length,
+ copy_bit_length,
type_byte_order (type ()) == BFD_ENDIAN_BIG);

/* Copy the meta-data. */