From 03731b326a80b582e48d939fe62cb1e2b10400d9 Mon Sep 17 00:00:00 2001 From: Olivier Fourdan Date: Mon, 7 Apr 2025 16:13:34 +0200 Subject: [PATCH] os: Do not overflow the integer size with BigRequest MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The BigRequest extension allows requests larger than the 16-bit length limit. It uses integers for the request length and checks for the size not to exceed the maxBigRequestSize limit, but does so after translating the length to integer by multiplying the given size in bytes by 4. In doing so, it might overflow the integer size limit before actually checking for the overflow, defeating the purpose of the test. To avoid the issue, make sure to check that the request size does not overflow the maxBigRequestSize limit prior to any conversion. The caller Dispatch() function however expects the return value to be in bytes, so we cannot just return the converted value in case of error, as that would also overflow the integer size. To preserve the existing API, we use a negative value for the X11 error code BadLength as the function only return positive values, 0 or -1 and update the caller Dispatch() function to take that case into account to return the error code to the offending client. CVE-2025-49176 This issue was discovered by Nils Emmerich and reported by Julian Suleder via ERNW Vulnerability Disclosure. Signed-off-by: Olivier Fourdan Reviewed-by: Michel Dänzer Part-of: --- dix/dispatch.c | 9 +++++---- os/io.c | 4 ++++ 2 files changed, 9 insertions(+), 4 deletions(-) diff --git a/dix/dispatch.c b/dix/dispatch.c index b3e5feacc2..2308cfe6d1 100644 --- a/dix/dispatch.c +++ b/dix/dispatch.c @@ -527,9 +527,10 @@ Dispatch(void) /* now, finally, deal with client requests */ result = ReadRequestFromClient(client); - if (result <= 0) { - if (result < 0) - CloseDownClient(client); + if (result == 0) + break; + else if (result == -1) { + CloseDownClient(client); break; } @@ -550,7 +551,7 @@ Dispatch(void) client->index, client->requestBuffer); #endif - if (result > (maxBigRequestSize << 2)) + if (result < 0 || result > (maxBigRequestSize << 2)) result = BadLength; else { result = XaceHookDispatch(client, client->majorOp); diff --git a/os/io.c b/os/io.c index 1fffaf62c7..3e39c10e6f 100644 --- a/os/io.c +++ b/os/io.c @@ -300,6 +300,10 @@ ReadRequestFromClient(ClientPtr client) needed = get_big_req_len(request, client); } client->req_len = needed; + if (needed > MAXINT >> 2) { + /* Check for potential integer overflow */ + return -(BadLength); + } needed <<= 2; /* needed is in bytes now */ } if (gotnow < needed) { -- GitLab