[Qemu-devel] [PATCH v2 2/9] ccid-card-passthru: Replace never trigger if statement by an assertion

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

The right side of the comparison is the return value of can_read():
VSCARD_IN_SIZE - card->vscard_in_pos.
Since the 'size' argument of chardev::read() is bound to
what chardev::can_read() returns, this condition can never happen.

Add an assertion, which will always fail if card->vscard_in_pos >=
VSCARD_IN_SIZE), since size > 0.

This is a quick fix for CVE-2018-18438 "Integer overflow in
ccid_card_vscard_read() allows memory corruption".

Fixes: CVE-2018-18438
Reported-by: Arash Tohidi Chafi <tohidi.arash@gmail.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 hw/usb/ccid-card-passthru.c | 14 +-------------
 1 file changed, 1 insertion(+), 13 deletions(-)

diff --git a/hw/usb/ccid-card-passthru.c b/hw/usb/ccid-card-passthru.c
index 8bb1314f49..1676b5fc05 100644
--- a/hw/usb/ccid-card-passthru.c
+++ b/hw/usb/ccid-card-passthru.c
@@ -264,24 +264,12 @@ static void ccid_card_vscard_handle_message(PassthruState *card,
     }
 }
 
-static void ccid_card_vscard_drop_connection(PassthruState *card)
-{
-    qemu_chr_fe_deinit(&card->cs, true);
-    card->vscard_in_pos = card->vscard_in_hdr = 0;
-}
-
 static void ccid_card_vscard_read(void *opaque, const uint8_t *buf, int size)
 {
     PassthruState *card = opaque;
     VSCMsgHeader *hdr;
 
-    if (card->vscard_in_pos + size > VSCARD_IN_SIZE) {
-        error_report("no room for data: pos %u +  size %d > %" PRId64 "."
-                     " dropping connection.",
-                     card->vscard_in_pos, size, VSCARD_IN_SIZE);
-        ccid_card_vscard_drop_connection(card);
-        return;
-    }
+    assert(size <= VSCARD_IN_SIZE - card->vscard_in_pos);
     assert(card->vscard_in_hdr < VSCARD_IN_SIZE);
     memcpy(card->vscard_in_data + card->vscard_in_pos, buf, size);
     card->vscard_in_pos += size;
-- 
2.20.1