The global QoS list 'qos_head' in net/atm/mpc.c is accessed from the /proc/net/atm/mpc procfs interface without proper synchronization. The read-side seq_file show path (mpc_show() -> atm_mpoa_disp_qos()) walks qos_head without any lock, while the write-side path (proc_mpc_write() -> parse_qos() -> atm_mpoa_delete_qos()) can unlink and kfree() entries immediately. Concurrent read/write therefore leads to a use-after-free.

This risk is already called out in-tree: /* this is buggered - we need locking for qos_head */

Fix this by adding a mutex to protect all qos_head list operations. A mutex is used (instead of a spinlock) because atm_mpoa_disp_qos() invokes seq_printf(), which may sleep.

KASAN report:

================================================================== [ 15.911538] BUG: KASAN: slab-use-after-free in atm_mpoa_disp_qos+0x202/0x380 [ 15.911988] Read of size 8 at addr ffff888007517380 by task mpoa_uaf_poc/89 [ 15.912123] [ 15.912717] CPU: 0 UID: 0 PID: 89 Comm: mpoa_uaf_poc Not tainted 6.17.8 #1 PREEMPT(voluntary) [ 15.912950] Hardware name: QEMU Ubuntu 25.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 [ 15.913110] Call Trace: [ 15.913167] <TASK> [ 15.913247] dump_stack_lvl+0x4e/0x70 [ 15.913343] ? atm_mpoa_disp_qos+0x202/0x380 [ 15.913364] print_report+0x174/0x4f6 [ 15.913386] ? __pfx__raw_spin_lock_irqsave+0x10/0x10 [ 15.913412] ? atm_mpoa_disp_qos+0x202/0x380 [ 15.913430] kasan_report+0xce/0x100 [ 15.913453] ? atm_mpoa_disp_qos+0x202/0x380 [ 15.913475] atm_mpoa_disp_qos+0x202/0x380 [ 15.913496] mpc_show+0x575/0x700 [ 15.913515] ? kasan_save_track+0x14/0x30 [ 15.913532] ? __pfx_mpc_show+0x10/0x10 [ 15.913550] ? __pfx_mutex_lock+0x10/0x10 [ 15.913565] ? seq_read_iter+0x697/0x1110 [ 15.913584] seq_read_iter+0x2bc/0x1110 [ 15.913607] seq_read+0x267/0x3d0 [ 15.913623] ? __pfx_seq_read+0x10/0x10 [ 15.913650] proc_reg_read+0x1ab/0x270 [ 15.913669] vfs_read+0x175/0xa10 [ 15.913687] ? do_sys_openat2+0x103/0x170 [ 15.913701] ? kmem_cache_free+0xc4/0x360 [ 15.913718] ? getname_flags.part.0+0xf3/0x470 [ 15.913734] ? __pfx_vfs_read+0x10/0x10 [ 15.913751] ? mutex_lock+0x81/0xe0 [ 15.913766] ? __pfx_mutex_lock+0x10/0x10 [ 15.913782] ? __rseq_handle_notify_resume+0x4c4/0xac0 [ 15.913802] ? fdget_pos+0x24d/0x4b0 [ 15.913829] ksys_read+0xf7/0x1c0 [ 15.913850] ? __pfx_ksys_read+0x10/0x10 [ 15.913877] do_syscall_64+0xa4/0x290 [ 15.913917] entry_SYSCALL_64_after_hwframe+0x77/0x7f [ 15.913981] RIP: 0033:0x45c982 [ 15.914171] Code: 08 0f 85 71 ea ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 f3 0f 1e fa 55 48 89 e5 [ 15.914239] RSP: 002b:00007f4b2b2cb0d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 [ 15.914315] RAX: ffffffffffffffda RBX: 00007f4b2b2cc6c0 RCX: 000000000045c982 [ 15.914352] RDX: 0000000000000fff RSI: 00007f4b2b2cb220 RDI: 0000000000000014 [ 15.914382] RBP: 00007f4b2b2cb100 R08: 0000000000000000 R09: 0000000000000000 [ 15.914413] R10: 0000000000000000 R11: 0000000000000246 R12: 000000000000000a [ 15.914445] R13: ffffffffffffffd0 R14: 0000000000000000 R15: 00007ffd386450c0 [ 15.914483] </TASK> [ 15.914533] [ 15.916812] Allocated by task 78: [ 15.916975] kasan_save_stack+0x30/0x50 [ 15.917047] kasan_save_track+0x14/0x30 [ 15.917105] __kasan_kmalloc+0x8f/0xa0 [ 15.917162] atm_mpoa_add_qos+0x1bb/0x3c0 [ 15.917220] parse_qos.cold+0x73/0x7d [ 15.917276] proc_mpc_write+0xf4/0x150 [ 15.917331] proc_reg_write+0x1ab/0x270 [ 15.917379] vfs_write+0x1ce/0xd30 [ 15.917431] ksys_write+0xf7/0x1c0 [ 15.917476] do_syscall_64+0xa4/0x290 [ 15.917523] entry_SYSCALL_64_after_hwframe+0x77/0x7f [ 15.917586] [ 15.917628] Freed by task 78: [ 15.917665] kasan_save_stack+0x30/0x50 [ 15.917714] kasan_save_track+0x14/0x30 [ 15.917762] kasan_save_free_info+0x3b/0x70 [ 15.917811] __kasan_slab_free+0x3e/0x50 [ 15.918058] kfree+0x121/0x340 [ 15.918135] atm_mpoa_delete_qos+0xad/0xd0 [ 15.918196] parse_qos+0x1e5/0x1f0 [ 15.918339] proc_mpc_write+0xf4/0x150 [ 15.918438] proc_reg_write+0x1ab/0x270 [ 15.918494] vfs_write+0x1ce/0xd30 [ 15.918538] ksys_write+0xf7/0x1c0 [ 15.918589] do_syscall_64+0xa4/0x290 [ 15.918634] entry_SYSCALL_64_after_hwframe+0x77/0x7f [ 15.918694] [ 15.918751] The buggy address belongs to the object at ffff888007517380 [ 15.918751] which belongs to the cache kmalloc-96 of size 96 [ 15.918911] The buggy address is located 0 bytes inside of [ 15.918911] freed 96-byte region [ffff888007517380, ffff8880075173e0) [ 15.919027] [ 15.919101] The buggy address belongs to the physical page: [ 15.919416] page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff888007517300 pfn:0x7517 [ 15.919679] anon flags: 0x100000000000000(node=0|zone=1) [ 15.920064] page_type: f5(slab) [ 15.920361] raw: 0100000000000000 ffff888006c41280 0000000000000000 dead000000000001 [ 15.920460] raw: ffff888007517300 0000000080200007 00000000f5000000 0000000000000000 [ 15.920577] page dumped because: kasan: bad access detected [ 15.920634] [ 15.920663] Memory state around the buggy address: [ 15.920860] ffff888007517280: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc [ 15.920944] ffff888007517300: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc [ 15.921017] >ffff888007517380: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc [ 15.921088] ^ [ 15.921163] ffff888007517400: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc [ 15.921222] ffff888007517480: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc [ 15.921313] ==================================================================

Reported-by: Minseong Kim ii4gsp@gmail.com Closes: https://lore.kernel.org/netdev/CAKrymDR1X3XTX_1ZW3XXXnuYH+kzsnv7Av5uivzR1sto... Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: stable@vger.kernel.org Signed-off-by: Minseong Kim ii4gsp@gmail.com --- net/atm/mpc.c | 117 ++++++++++++++++++++++++++++++++++++-------------- 1 file changed, 85 insertions(+), 32 deletions(-)

diff --git a/net/atm/mpc.c b/net/atm/mpc.c index f6b447bba329..c268b5f4a266 100644 --- a/net/atm/mpc.c +++ b/net/atm/mpc.c @@ -9,6 +9,7 @@ #include <linux/bitops.h> #include <linux/capability.h> #include <linux/seq_file.h> +#include <linux/mutex.h>

/* We are an ethernet device */ #include <linux/if_ether.h> @@ -122,6 +123,7 @@ static struct notifier_block mpoa_notifier = {

struct mpoa_client *mpcs = NULL; /* FIXME */ static struct atm_mpoa_qos *qos_head = NULL; +static DEFINE_MUTEX(qos_mutex); /* Protect qos_head list */ static DEFINE_TIMER(mpc_timer, mpc_cache_check);

@@ -171,74 +173,120 @@ static struct mpoa_client *find_mpc_by_lec(struct net_device *dev) * Functions for managing QoS list */

+/* + * Search for a QoS entry. Caller must hold qos_mutex. + * Returns pointer to entry if found, NULL otherwise. + */ +static struct atm_mpoa_qos *__atm_mpoa_search_qos(__be32 dst_ip) +{ + struct atm_mpoa_qos *qos = qos_head; + + while (qos) { + if (qos->ipaddr == dst_ip) + return qos; + qos = qos->next; + } + return NULL; +} + +/* + * Search for a QoS entry. + * WARNING: The returned pointer is not protected. The caller must ensure + * that the entry is not freed while using it, or hold qos_mutex during use. + */ +struct atm_mpoa_qos *atm_mpoa_search_qos(__be32 dst_ip) +{ + struct atm_mpoa_qos *qos; + + mutex_lock(&qos_mutex); + qos = __atm_mpoa_search_qos(dst_ip); + mutex_unlock(&qos_mutex); + + return qos; +} + /* * Overwrites the old entry or makes a new one. */ struct atm_mpoa_qos *atm_mpoa_add_qos(__be32 dst_ip, struct atm_qos *qos) { struct atm_mpoa_qos *entry; + struct atm_mpoa_qos *new;

- entry = atm_mpoa_search_qos(dst_ip); - if (entry != NULL) { + /* Fast path: update existing entry */ + mutex_lock(&qos_mutex); + entry = __atm_mpoa_search_qos(dst_ip); + if (entry) { entry->qos = *qos; + mutex_unlock(&qos_mutex); return entry; } + mutex_unlock(&qos_mutex);

- entry = kmalloc(sizeof(struct atm_mpoa_qos), GFP_KERNEL); - if (entry == NULL) { + /* Allocate outside lock */ + new = kmalloc(sizeof(*new), GFP_KERNEL); + if (!new) { pr_info("mpoa: out of memory\n"); - return entry; + return NULL; }

- entry->ipaddr = dst_ip; - entry->qos = *qos; + new->ipaddr = dst_ip; + new->qos = *qos;

- entry->next = qos_head; - qos_head = entry; - - return entry; -} - -struct atm_mpoa_qos *atm_mpoa_search_qos(__be32 dst_ip) -{ - struct atm_mpoa_qos *qos; - - qos = qos_head; - while (qos) { - if (qos->ipaddr == dst_ip) - break; - qos = qos->next; + /* Re-check under lock to avoid duplicates */ + mutex_lock(&qos_mutex); + entry = __atm_mpoa_search_qos(dst_ip); + if (entry) { + entry->qos = *qos; + mutex_unlock(&qos_mutex); + kfree(new); + return entry; }

- return qos; + new->next = qos_head; + qos_head = new; + mutex_unlock(&qos_mutex); + + return new; }

/* - * Returns 0 for failure + * Returns 0 for failure, 1 for success */ int atm_mpoa_delete_qos(struct atm_mpoa_qos *entry) { struct atm_mpoa_qos *curr; + int ret = 0;

if (entry == NULL) return 0; + + mutex_lock(&qos_mutex); + if (entry == qos_head) { qos_head = qos_head->next; - kfree(entry); - return 1; + ret = 1; + goto out_free; }

curr = qos_head; - while (curr != NULL) { + while (curr) { if (curr->next == entry) { curr->next = entry->next; - kfree(entry); - return 1; + ret = 1; + goto out_free; } curr = curr->next; }

- return 0; +out: + mutex_unlock(&qos_mutex); + return ret; + +out_free: + mutex_unlock(&qos_mutex); + kfree(entry); + return ret; }

/* this is buggered - we need locking for qos_head */ @@ -246,10 +294,12 @@ void atm_mpoa_disp_qos(struct seq_file *m) { struct atm_mpoa_qos *qos;

- qos = qos_head; seq_printf(m, "QoS entries for shortcuts:\n"); - seq_printf(m, "IP address\n TX:max_pcr pcr min_pcr max_cdv max_sdu\n RX:max_pcr pcr min_pcr max_cdv max_sdu\n"); + seq_printf(m, "IP address\n TX:max_pcr pcr min_pcr max_cdv max_sdu\n" + " RX:max_pcr pcr min_pcr max_cdv max_sdu\n");

+ mutex_lock(&qos_mutex); + qos = qos_head; while (qos != NULL) { seq_printf(m, "%pI4\n %-7d %-7d %-7d %-7d %-7d\n %-7d %-7d %-7d %-7d %-7d\n", &qos->ipaddr, @@ -265,6 +315,7 @@ void atm_mpoa_disp_qos(struct seq_file *m) qos->qos.rxtp.max_sdu); qos = qos->next; } + mutex_unlock(&qos_mutex); }

static struct net_device *find_lec_by_itfnum(int itf) @@ -1521,8 +1572,10 @@ static void __exit atm_mpoa_cleanup(void) mpc = tmp; }

+ mutex_lock(&qos_mutex); qos = qos_head; qos_head = NULL; + mutex_unlock(&qos_mutex); while (qos != NULL) { nextqos = qos->next; dprintk("freeing qos entry %p\n", qos);