From: Gustavo Luiz Duarte Date: Tue, 28 Oct 2025 22:06:32 +0000 (-0700) Subject: netconsole: Fix race condition in between reader and writer of userdata X-Git-Tag: v6.18-rc4~24^2~11 X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=00764aa5c9bbb2044eb04d6d78584a436666b231;p=thirdparty%2Fkernel%2Flinux.git netconsole: Fix race condition in between reader and writer of userdata The update_userdata() function constructs the complete userdata string in nt->extradata_complete and updates nt->userdata_length. This data is then read by write_msg() and write_ext_msg() when sending netconsole messages. However, update_userdata() was not holding target_list_lock during this process, allowing concurrent message transmission to read partially updated userdata. This race condition could result in netconsole messages containing incomplete or inconsistent userdata - for example, reading the old userdata_length with new extradata_complete content, or vice versa, leading to truncated or corrupted output. Fix this by acquiring target_list_lock with spin_lock_irqsave() before updating extradata_complete and userdata_length, and releasing it after both fields are fully updated. This ensures that readers see a consistent view of the userdata, preventing corruption during concurrent access. The fix aligns with the existing locking pattern used throughout the netconsole code, where target_list_lock protects access to target fields including buf[] and msgcounter that are accessed during message transmission. Also get rid of the unnecessary variable complete_idx, which makes it easier to bail out of update_userdata(). Fixes: df03f830d099 ("net: netconsole: cache userdata formatted string in netconsole_target") Signed-off-by: Gustavo Luiz Duarte Link: https://patch.msgid.link/20251028-netconsole-fix-race-v4-1-63560b0ae1a0@meta.com Signed-off-by: Jakub Kicinski --- diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c index 194570443493..5d8d0214786c 100644 --- a/drivers/net/netconsole.c +++ b/drivers/net/netconsole.c @@ -886,8 +886,11 @@ static ssize_t userdatum_value_show(struct config_item *item, char *buf) static void update_userdata(struct netconsole_target *nt) { - int complete_idx = 0, child_count = 0; struct list_head *entry; + int child_count = 0; + unsigned long flags; + + spin_lock_irqsave(&target_list_lock, flags); /* Clear the current string in case the last userdatum was deleted */ nt->userdata_length = 0; @@ -897,8 +900,11 @@ static void update_userdata(struct netconsole_target *nt) struct userdatum *udm_item; struct config_item *item; - if (WARN_ON_ONCE(child_count >= MAX_EXTRADATA_ITEMS)) - break; + if (child_count >= MAX_EXTRADATA_ITEMS) { + spin_unlock_irqrestore(&target_list_lock, flags); + WARN_ON_ONCE(1); + return; + } child_count++; item = container_of(entry, struct config_item, ci_entry); @@ -912,12 +918,11 @@ static void update_userdata(struct netconsole_target *nt) * one entry length (1/MAX_EXTRADATA_ITEMS long), entry count is * checked to not exceed MAX items with child_count above */ - complete_idx += scnprintf(&nt->extradata_complete[complete_idx], - MAX_EXTRADATA_ENTRY_LEN, " %s=%s\n", - item->ci_name, udm_item->value); + nt->userdata_length += scnprintf(&nt->extradata_complete[nt->userdata_length], + MAX_EXTRADATA_ENTRY_LEN, " %s=%s\n", + item->ci_name, udm_item->value); } - nt->userdata_length = strnlen(nt->extradata_complete, - sizeof(nt->extradata_complete)); + spin_unlock_irqrestore(&target_list_lock, flags); } static ssize_t userdatum_value_store(struct config_item *item, const char *buf,