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.
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| @@ -261,6 +261,7 @@ EXPORT_SYMBOL_GPL(param_set_uint_minmax); · 260 unmodified lines | |||
| 261 | 261 | ||
| 262 | int param_set_charp(const char *val, const struct kernel_param *kp) | 262 | int param_set_charp(const char *val, const struct kernel_param *kp) |
| 263 | { | 263 | { |
| 264 | char *tmp; | ||
| 264 | size_t len, maxlen = 1024; | 265 | size_t len, maxlen = 1024; |
| 265 | 266 | ||
| 266 | len = strnlen(val, maxlen + 1); | 267 | len = strnlen(val, maxlen + 1); |
| @@ -269,19 +270,20 @@ int param_set_charp(const char *val, const struct kernel_param *kp) · 2 unmodified lines | |||
| 269 | return -ENOSPC; | 270 | return -ENOSPC; |
| 270 | } | 271 | } |
| 271 | 272 | ||
| 272 | maybe_kfree_parameter(*(char **)kp->arg); | ||
| 273 | |||
| 274 | /* | 273 | /* |
| 275 | * This is a hack. We can't kmalloc() in early boot, and we | 274 | * This is a hack. We can't kmalloc() in early boot, and we |
| 276 | * don't need to; this mangled commandline is preserved. | 275 | * don't need to; this mangled commandline is preserved. |
| 277 | */ | 276 | */ |
| 278 | if (slab_is_available()) { | 277 | if (slab_is_available()) { |
| 279 | *(char **)kp->arg = kmalloc_parameter(len + 1); | 278 | tmp = kmalloc_parameter(len + 1); |
| 280 | if (!*(char **)kp->arg) | 279 | if (!tmp) |
| 281 | return -ENOMEM; | 280 | return -ENOMEM; |
| 282 | strcpy(*(char **)kp->arg, val); | 281 | memcpy(tmp, val, len + 1); |
| 283 | } else | 282 | } else |
| 284 | *(const char **)kp->arg = val; | 283 | tmp = (char *)val; |
| 284 | |||
| 285 | maybe_kfree_parameter(*(char **)kp->arg); | ||
| 286 | *(char **)kp->arg = tmp; | ||
| 285 | 287 | ||
| 286 | return 0; | 288 | return 0; |
| 287 | } | 289 | } |