From c8bb46c5a16ed02401f4a0b46c74f0f46c1b6434 Mon Sep 17 00:00:00 2001
From: Lasse Collin <lasse.collin@tukaani.org>
Date: Thu, 3 Apr 2025 14:34:42 +0300
Subject: [PATCH 4/4] liblzma: mt dec: Don't modify thr->in_size in the worker
 thread

Don't set thr->in_size = 0 when returning the thread to the stack of
available threads. Not only is it useless, but the main thread may
read the value in SEQ_BLOCK_THR_RUN. With valid inputs, it made
no difference if the main thread saw the original value or 0. With
invalid inputs (when worker thread stops early), thr->in_size was
no longer modified after the previous commit with the security fix
("Don't free the input buffer too early").

So while the bug appears harmless now, it's important to fix it because
the variable was being modified without proper locking. It's trivial
to fix because there is no need to change the value. Only main thread
needs to set the value in (in SEQ_BLOCK_THR_INIT) when starting a new
Block before the worker thread is activated.

Fixes: 4cce3e27f529 ("liblzma: Add threaded .xz decompressor.")
Reviewed-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Thanks-to: Sam James <sam@gentoo.org>
(cherry picked from commit 8188048854e8d11071b8a50d093c74f4c030acc9)

CVE: CVE-2025-31115
Upstream-Status: Backport [https://github.com/tukaani-project/xz/commit/c8bb46c5a16ed02401f4a0b46c74f0f46c1b6434]
Signed-off-by: Peter Marko <peter.marko@siemens.com>
---
 src/liblzma/common/stream_decoder_mt.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/liblzma/common/stream_decoder_mt.c b/src/liblzma/common/stream_decoder_mt.c
index 259c4c65..6bbbe53b 100644
--- a/src/liblzma/common/stream_decoder_mt.c
+++ b/src/liblzma/common/stream_decoder_mt.c
@@ -492,8 +492,6 @@ next_loop_unlocked:
 		if (ret == LZMA_STREAM_END) {
 			// Update memory usage counters.
 			thr->coder->mem_in_use -= thr->in_size;
-			thr->in_size = 0; // thr->in was freed above.
-
 			thr->coder->mem_in_use -= thr->mem_filters;
 			thr->coder->mem_cached += thr->mem_filters;
 
@@ -1558,6 +1556,10 @@ stream_decode_mt(void *coder_ptr, const lzma_allocator *allocator,
 		}
 
 		// Return if the input didn't contain the whole Block.
+		//
+		// NOTE: When we updated coder->thr->in_filled a few lines
+		// above, the worker thread might by now have finished its
+		// work and returned itself back to the stack of free threads.
 		if (coder->thr->in_filled < coder->thr->in_size) {
 			assert(*in_pos == in_size);
 			return LZMA_OK;
