dup_task_struct() copies the parent's task_struct, including ret_stack.
ftrace_graph_init_task() clears the copied function graph state, but it
currently runs after copy_exec_state().
For non-CLONE_VM forks, copy_exec_state() allocates a new task_exec_state.
If that allocation fails, copy_process() reaches bad_fork_free and
free_task() calls ftrace_graph_exit_task(). Since the child still carries
the parent's ret_stack pointer, the unwind frees the parent's active
function graph return stack. The parent subsequently accesses freed memory
from function_graph_enter_regs().
KASAN reports:
[ 22.190920]
==================================================================
[ 22.195899] BUG: KASAN: slab-use-after-free in
function_graph_enter_regs+0xa76/0xb90
[ 22.200747] Write of size 8 at addr ff110000054dc0a8 by task repro/1
[ 22.205134]
[ 22.210770] CPU: 0 UID: 0 PID: 1 Comm: repro Not tainted
7.2.0-07732-g9328b3b03bdc-dirty #3 PREEMPT(lazy)
[ 22.212576] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.16.3-debian-1.16.3-2 04/01/2014
[ 22.213750] Call Trace:
[ 22.215271] <TASK>
[ 22.216242] ? ftrace_stub_direct_tramp+0x10/0x10
[ 22.217774] dump_stack_lvl+0x4e/0x70
[ 22.220531] print_report+0x157/0x4b4
[ 22.223202] ? fixup_red_left+0x9/0x30
[ 22.224407] ? complete_report_info+0x83/0x110
[ 22.226679] ? function_graph_enter_regs+0xa76/0xb90
[ 22.228084] kasan_report+0xce/0x100
[ 22.230109] ? function_graph_enter_regs+0xa76/0xb90
[ 22.232860] ? stack_trace_save+0x4/0xd0
[ 22.234156] function_graph_enter_regs+0xa76/0xb90
[ 22.236090] ? kasan_save_stack+0x30/0x50
[ 22.237752] ? __pfx_function_graph_enter_regs+0x10/0x10
[ 22.238694] ? ring_buffer_lock_reserve+0x345/0xf80
[ 22.239628] ? stack_trace_save+0x4/0xd0
[ 22.242121] ? stack_trace_save+0x4/0xd0
[ 22.243588] ftrace_graph_func+0xda/0x160
[ 22.245362] ? ftrace_stub_direct_tramp+0x10/0x10
[ 22.246520] 0xffffffffa0000095
[ 22.250528] ? stack_trace_save+0x9/0xd0
[ 22.251757] ? ring_buffer_unlock_commit+0x11d/0x5c0
[ 22.253152] stack_trace_save+0x9/0xd0
[ 22.254264] kasan_save_stack+0x30/0x50
[ 22.273631] kasan_save_track+0x14/0x30
[ 22.276763] kasan_save_free_info+0x3b/0x70
[ 22.278296] __kasan_slab_free+0x43/0x70
[ 22.280157] kmem_cache_free+0xbf/0x3b0
[ 22.282963] ? ftrace_stub_direct_tramp+0x10/0x10
[ 22.284001] free_task+0xa2/0x160
[ 22.285699] ? ftrace_stub_direct_tramp+0x10/0x10
[ 22.286752] copy_process+0x2aae/0x7bc0
Initialize the child function graph state immediately after
dup_task_struct(), before the first fallible operation.
Fixes: 6b1c66c9cca9 ("exec_state: relocate dumpable information")
Cc: [email protected]
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <[email protected]>
---
Resending v2 as requested by adding Cc: [email protected]
Changes in v2:
- Add the KASAN report excerpt to the commit message.
- Add a comment documenting why ftrace_graph_init_task() must run before
fallible initialization.
v1:
https://lore.kernel.org/all/[email protected]/
kernel/fork.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index 1e68404bd773..0d1ad92e2d33 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2139,6 +2139,11 @@ __latent_entropy struct task_struct *copy_process(
p = dup_task_struct(current, node);
if (!p)
goto fork_out;
+ /*
+ * Must run before the first fallible op, so error paths never
+ * free the parent's ret_stack.
+ */
+ ftrace_graph_init_task(p);
retval = copy_exec_state(clone_flags, p);
if (retval)
goto bad_fork_free;
@@ -2165,8 +2170,6 @@ __latent_entropy struct task_struct *copy_process(
*/
p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ?
args->child_tid : NULL;
- ftrace_graph_init_task(p);
-
rt_mutex_init_task(p);
raw_spin_lock_init(&p->blocked_lock);
--
2.47.3
