Bug 2388932 (CVE-2025-38525)

Summary:	CVE-2025-38525 kernel: rxrpc: Fix irq-disabled in local_bh_enable()
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security DevOps Team <prodsec-dev>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	Keywords:	Security
Target Milestone:	---
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	---
Doc Text:	A flaw was found in the Linux kernel’s Remote Execution RPC (RxRPC) implementation, where the rxrpc_assess_MTU_size() function is invoked with interrupts disabled during the handling of an incoming call. This function queries the IP layer for route MTU information, but the IP layer internally uses local_bh_enable(), which can raise warnings if executed while IRQs are disabled. As a result, a race condition could occur, leading to kernel warnings and potential instability.	Story Points:	---
Clone Of:		Environment:
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2025-08-16 12:02:21 UTC

In the Linux kernel, the following vulnerability has been resolved:

rxrpc: Fix irq-disabled in local_bh_enable()

The rxrpc_assess_MTU_size() function calls down into the IP layer to find
out the MTU size for a route.  When accepting an incoming call, this is
called from rxrpc_new_incoming_call() which holds interrupts disabled
across the code that calls down to it.  Unfortunately, the IP layer uses
local_bh_enable() which, config dependent, throws a warning if IRQs are
enabled:

WARNING: CPU: 1 PID: 5544 at kernel/softirq.c:387 __local_bh_enable_ip+0x43/0xd0
...
RIP: 0010:__local_bh_enable_ip+0x43/0xd0
...
Call Trace:
 <TASK>
 rt_cache_route+0x7e/0xa0
 rt_set_nexthop.isra.0+0x3b3/0x3f0
 __mkroute_output+0x43a/0x460
 ip_route_output_key_hash+0xf7/0x140
 ip_route_output_flow+0x1b/0x90
 rxrpc_assess_MTU_size.isra.0+0x2a0/0x590
 rxrpc_new_incoming_peer+0x46/0x120
 rxrpc_alloc_incoming_call+0x1b1/0x400
 rxrpc_new_incoming_call+0x1da/0x5e0
 rxrpc_input_packet+0x827/0x900
 rxrpc_io_thread+0x403/0xb60
 kthread+0x2f7/0x310
 ret_from_fork+0x2a/0x230
 ret_from_fork_asm+0x1a/0x30
...
hardirqs last  enabled at (23): _raw_spin_unlock_irq+0x24/0x50
hardirqs last disabled at (24): _raw_read_lock_irq+0x17/0x70
softirqs last  enabled at (0): copy_process+0xc61/0x2730
softirqs last disabled at (25): rt_add_uncached_list+0x3c/0x90

Fix this by moving the call to rxrpc_assess_MTU_size() out of
rxrpc_init_peer() and further up the stack where it can be done without
interrupts disabled.

It shouldn't be a problem for rxrpc_new_incoming_call() to do it after the
locks are dropped as pmtud is going to be performed by the I/O thread - and
we're in the I/O thread at this point.

Comment 1 Sandipan Roy 2025-08-18 05:05:53 UTC

Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025081653-CVE-2025-38525-f516@gregkh/T