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Subject[PATCH] mm/hugetlb: Fix a race between hugetlb sysctl handlers
Date
There is a race between the assignment of `table->data` and write value
to the pointer of `table->data` in the __do_proc_doulongvec_minmax().
Fix this by duplicating the `table`, and only update the duplicate of
it. And introduce a helper of proc_hugetlb_doulongvec_minmax() to
simplify the code.

The following oops was seen:

BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor instruction fetch in kernel mode
#PF: error_code(0x0010) - not-present page
Code: Bad RIP value.
...
Call Trace:
? set_max_huge_pages+0x3da/0x4f0
? alloc_pool_huge_page+0x150/0x150
? proc_doulongvec_minmax+0x46/0x60
? hugetlb_sysctl_handler_common+0x1c7/0x200
? nr_hugepages_store+0x20/0x20
? copy_fd_bitmaps+0x170/0x170
? hugetlb_sysctl_handler+0x1e/0x20
? proc_sys_call_handler+0x2f1/0x300
? unregister_sysctl_table+0xb0/0xb0
? __fd_install+0x78/0x100
? proc_sys_write+0x14/0x20
? __vfs_write+0x4d/0x90
? vfs_write+0xef/0x240
? ksys_write+0xc0/0x160
? __ia32_sys_read+0x50/0x50
? __close_fd+0x129/0x150
? __x64_sys_write+0x43/0x50
? do_syscall_64+0x6c/0x200
? entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: e5ff215941d5 ("hugetlb: multiple hstates for multiple page sizes")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
mm/hugetlb.c | 27 +++++++++++++++++++++------
1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a301c2d672bf..818d6125af49 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3454,6 +3454,23 @@ static unsigned int allowed_mems_nr(struct hstate *h)
}

#ifdef CONFIG_SYSCTL
+static int proc_hugetlb_doulongvec_minmax(struct ctl_table *table, int write,
+ void *buffer, size_t *length,
+ loff_t *ppos, unsigned long *out)
+{
+ struct ctl_table dup_table;
+
+ /*
+ * In order to avoid races with __do_proc_doulongvec_minmax(), we
+ * can duplicate the @table and alter the duplicate of it.
+ */
+ dup_table = *table;
+ dup_table.data = out;
+ dup_table.maxlen = sizeof(unsigned long);
+
+ return proc_doulongvec_minmax(&dup_table, write, buffer, length, ppos);
+}
+
static int hugetlb_sysctl_handler_common(bool obey_mempolicy,
struct ctl_table *table, int write,
void *buffer, size_t *length, loff_t *ppos)
@@ -3465,9 +3482,8 @@ static int hugetlb_sysctl_handler_common(bool obey_mempolicy,
if (!hugepages_supported())
return -EOPNOTSUPP;

- table->data = &tmp;
- table->maxlen = sizeof(unsigned long);
- ret = proc_doulongvec_minmax(table, write, buffer, length, ppos);
+ ret = proc_hugetlb_doulongvec_minmax(table, write, buffer, length, ppos,
+ &tmp);
if (ret)
goto out;

@@ -3510,9 +3526,8 @@ int hugetlb_overcommit_handler(struct ctl_table *table, int write,
if (write && hstate_is_gigantic(h))
return -EINVAL;

- table->data = &tmp;
- table->maxlen = sizeof(unsigned long);
- ret = proc_doulongvec_minmax(table, write, buffer, length, ppos);
+ ret = proc_hugetlb_doulongvec_minmax(table, write, buffer, length, ppos,
+ &tmp);
if (ret)
goto out;

--
2.11.0
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