media: vidtv: fix NULL pointer dereference in vidtv_mux_push_si

syzbot reported a general protection fault in
vidtv_psi_ts_psi_write_into [1].

vidtv_mux_get_pid_ctx() can return NULL, but vidtv_mux_push_si() does not check for this before dereferencing the returned pointer to access the continuity counter. This leads to a general protection fault when accessing a near-NULL address.

The root cause is that vidtv_mux_pid_ctx_init() does not check the return value of vidtv_mux_create_pid_ctx_once() for PMT section PIDs. If the allocation fails, the PID context is never created, but init returns success. The subsequent vidtv_mux_push_si() call then gets NULL from vidtv_mux_get_pid_ctx() and crashes.

Fix both the root cause (add error check in vidtv_mux_pid_ctx_init for PMT PIDs) and add defensive NULL checks in vidtv_mux_push_si for all vidtv_mux_get_pid_ctx() calls.

[1]

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
Workqueue: events vidtv_mux_tick
RIP: 0010:vidtv_psi_ts_psi_write_into+0x54a/0xbc0 drivers/media/test-drivers/vidtv/vidtv_psi.c:197
Call Trace:
 <TASK>
 vidtv_psi_table_header_write_into drivers/media/test-drivers/vidtv/vidtv_psi.c:799 [inline]
 vidtv_psi_pmt_write_into+0x3b2/0xa70 drivers/media/test-drivers/vidtv/vidtv_psi.c:1231
 vidtv_mux_push_si+0x932/0xe80 drivers/media/test-drivers/vidtv/vidtv_mux.c:196
 vidtv_mux_tick+0xe9b/0x1480 drivers/media/test-drivers/vidtv/vidtv_mux.c:408

---
 drivers/media/test-drivers/vidtv/vidtv_mux.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

index 403fbedb86636..bc52f51418f25 100644
@@ -101,7 +101,8 @@ static int vidtv_mux_pid_ctx_init(struct vidtv_mux *m)
 	/* add a ctx for all PMT sections */
 	while (p) {
 		pid = vidtv_psi_get_pat_program_pid(p);
-		vidtv_mux_create_pid_ctx_once(m, pid);
+		if (!vidtv_mux_create_pid_ctx_once(m, pid))
+			goto free;
 		p = p->next;
 	}
 

@@ -170,6 +171,9 @@ static u32 vidtv_mux_push_si(struct vidtv_mux *m)
 	nit_ctx = vidtv_mux_get_pid_ctx(m, VIDTV_NIT_PID);
 	eit_ctx = vidtv_mux_get_pid_ctx(m, VIDTV_EIT_PID);
 
+	if (!pat_ctx || !sdt_ctx || !nit_ctx || !eit_ctx)
+		return 0;
+
 	pat_args.offset             = m->mux_buf_offset;
 	pat_args.continuity_counter = &pat_ctx->cc;
 

@@ -186,6 +190,8 @@ static u32 vidtv_mux_push_si(struct vidtv_mux *m)
 		}
 
 		pmt_ctx = vidtv_mux_get_pid_ctx(m, pmt_pid);
+		if (!pmt_ctx)
+			continue;
 
 		pmt_args.offset             = m->mux_buf_offset;
 		pmt_args.pmt                = m->si.pmt_secs[i];

2.43.0