[PATCH 6.1.y 6/6] params: fix charp corruption on allocation failure

[Sasha Levin <sashal@kernel.org>]

From: Jiacheng Yu <yujiacheng3@huawei.com>

[ Upstream commit 3dfaae04243cde460d82dfc2a7dd0bb6664d20ae ]

param_set_charp() stores charp parameters in allocated memory after slab is
available, and releases the previous value when the parameter is updated.

The previous value is released before the replacement allocation succeeds.
If kmalloc_parameter() fails, the setter returns -ENOMEM with the parameter
left as NULL.

Failing zswap's compressor update before zswap is initialized can later
trigger:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  RIP: 0010:strcmp+0x10/0x30
  Call Trace:
    zswap_setup+0x3b1/0x490
    zswap_enabled_param_set+0x5b/0xa0
    param_attr_store+0x93/0xe0
    module_attr_store+0x1c/0x30
    kernfs_fop_write_iter+0x116/0x1f0

Allocate and copy the replacement first, then replace the parameter value
only after allocation succeeds.

Fixes: e180a6b7759a ("param: fix charp parameters set via sysfs")
Cc: stable@vger.kernel.org
Signed-off-by: Jiacheng Yu <yujiacheng3@huawei.com>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/params.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/kernel/params.c b/kernel/params.c
index 03443c99496c6..1b699c7632598 100644
--- a/kernel/params.c
+++ b/kernel/params.c
@@ -265,6 +265,7 @@ EXPORT_SYMBOL_GPL(param_set_uint_minmax);
 
 int param_set_charp(const char *val, const struct kernel_param *kp)
 {
+	char *tmp;
 	size_t len, maxlen = 1024;
 
 	len = strnlen(val, maxlen + 1);
@@ -273,19 +274,20 @@ int param_set_charp(const char *val, const struct kernel_param *kp)
 		return -ENOSPC;
 	}
 
-	maybe_kfree_parameter(*(char **)kp->arg);
-
 	/*
 	 * This is a hack. We can't kmalloc() in early boot, and we
 	 * don't need to; this mangled commandline is preserved.
 	 */
 	if (slab_is_available()) {
-		*(char **)kp->arg = kmalloc_parameter(len + 1);
-		if (!*(char **)kp->arg)
+		tmp = kmalloc_parameter(len + 1);
+		if (!tmp)
 			return -ENOMEM;
-		strcpy(*(char **)kp->arg, val);
+		memcpy(tmp, val, len + 1);
 	} else
-		*(const char **)kp->arg = val;
+		tmp = (char *)val;
+
+	maybe_kfree_parameter(*(char **)kp->arg);
+	*(char **)kp->arg = tmp;
 
 	return 0;
 }
-- 
2.53.0