[PATCH v2 for-4.9 2/7] tools/insn-fuzz: Don't hit memcpy() for zero-length reads

[Andrew Cooper <andrew.cooper3@citrix.com>]

For control-flow changes, the emulator needs to perform a zero-length
instruction fetch at the target offset.  It also passes NULL for the
destination buffer, as there is no instruction stream to collect.

This trips up UBSAN when passed to memcpy(), as passing NULL is undefined
behaviour per the C spec (irrespective of passing a size of 0).

Special case these fetches in fuzz_insn_fetch() before reaching data_read().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>

v2:
 * Rework in terms of special casing zero-length fetches only.
---
 tools/fuzz/x86_instruction_emulator/fuzz-emul.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
index 65c5a3b..64b7fb2 100644
--- a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
+++ b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
@@ -117,6 +117,16 @@ static int fuzz_insn_fetch(
     unsigned int bytes,
     struct x86_emulate_ctxt *ctxt)
 {
+    /*
+     * Zero-length instruction fetches are made at the destination of jumps,
+     * to perform segmentation checks.  No data needs returning.
+     */
+    if ( bytes == 0 )
+    {
+        assert(p_data == NULL);
+        return maybe_fail("insn_fetch", true);
+    }
+
     return data_read("insn_fetch", p_data, bytes);
 }
 
-- 
2.1.4


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