From ce6d8bbdbb84cdbdc71b981af9b35a972f61de9a Mon Sep 17 00:00:00 2001 From: Joseph Sutton Date: Mon, 15 Aug 2022 16:53:45 +1200 Subject: [PATCH] gsskrb5: CVE-2022-3437 Avoid undefined behaviour in _gssapi_verify_pad() By decrementing 'pad' only when we know it's safe, we ensure we can't stray backwards past the start of a buffer, which would be undefined behaviour. In the previous version of the loop, 'i' is the number of bytes left to check, and 'pad' is the current byte we're checking. 'pad' was decremented at the end of each loop iteration. If 'i' was 1 (so we checked the final byte), 'pad' could potentially be pointing to the first byte of the input buffer, and the decrement would put it one byte behind the buffer. That would be undefined behaviour. The patch changes it so that 'pad' is the byte we previously checked, which allows us to ensure that we only decrement it when we know we have a byte to check. Samba BUG: https://bugzilla.samba.org/show_bug.cgi?id=15134 Signed-off-by: Joseph Sutton Reviewed-by: Andrew Bartlett --- lib/gssapi/krb5/decapsulate.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/lib/gssapi/krb5/decapsulate.c b/lib/gssapi/krb5/decapsulate.c index 86085f569..4e3fcd659 100644 --- a/lib/gssapi/krb5/decapsulate.c +++ b/lib/gssapi/krb5/decapsulate.c @@ -193,13 +193,13 @@ _gssapi_verify_pad(gss_buffer_t wrapped_token, if (wrapped_token->length < 1) return GSS_S_BAD_MECH; - pad = (u_char *)wrapped_token->value + wrapped_token->length - 1; - padlength = *pad; + pad = (u_char *)wrapped_token->value + wrapped_token->length; + padlength = pad[-1]; if (padlength > datalen) return GSS_S_BAD_MECH; - for (i = padlength; i > 0 && *pad == padlength; i--, pad--) + for (i = padlength; i > 0 && *--pad == padlength; i--) ; if (i != 0) return GSS_S_BAD_MIC;