In isdn_ppp_write(), the header (i.e., protobuf) of the buffer is fetched twice from userspace. The first fetch is used to peek at the protocol of the message and reset the huptimer if necessary; while the second fetch copies in the whole buffer. However, given that buf resides in userspace memory, a user process can race to change its memory content across fetches. By doing so, we can either avoid resetting the huptimer for any type of packets (by first setting proto to PPP_LCP and later change to the actual type) or force resetting the huptimer for LCP packets.
This patch changes this double-fetch behavior into two single fetches decided by condition (lp->isdn_device < 0 || lp->isdn_channel <0). A more detailed discussion can be found at https://marc.info/?l=linux-kernel&m=150586376926123&w=2 Signed-off-by: Meng Xu <mengxu.gat...@gmail.com> --- drivers/isdn/i4l/isdn_ppp.c | 37 +++++++++++++++++++++++++------------ 1 file changed, 25 insertions(+), 12 deletions(-) diff --git a/drivers/isdn/i4l/isdn_ppp.c b/drivers/isdn/i4l/isdn_ppp.c index 6c44609..cd2b3c6 100644 --- a/drivers/isdn/i4l/isdn_ppp.c +++ b/drivers/isdn/i4l/isdn_ppp.c @@ -825,7 +825,6 @@ isdn_ppp_write(int min, struct file *file, const char __user *buf, int count) isdn_net_local *lp; struct ippp_struct *is; int proto; - unsigned char protobuf[4]; is = file->private_data; @@ -839,24 +838,28 @@ isdn_ppp_write(int min, struct file *file, const char __user *buf, int count) if (!lp) printk(KERN_DEBUG "isdn_ppp_write: lp == NULL\n"); else { - /* - * Don't reset huptimer for - * LCP packets. (Echo requests). - */ - if (copy_from_user(protobuf, buf, 4)) - return -EFAULT; - proto = PPP_PROTOCOL(protobuf); - if (proto != PPP_LCP) - lp->huptimer = 0; + if (lp->isdn_device < 0 || lp->isdn_channel < 0) { + unsigned char protobuf[4]; + /* + * Don't reset huptimer for + * LCP packets. (Echo requests). + */ + if (copy_from_user(protobuf, buf, 4)) + return -EFAULT; + + proto = PPP_PROTOCOL(protobuf); + if (proto != PPP_LCP) + lp->huptimer = 0; - if (lp->isdn_device < 0 || lp->isdn_channel < 0) return 0; + } if ((dev->drv[lp->isdn_device]->flags & DRV_FLAG_RUNNING) && lp->dialstate == 0 && (lp->flags & ISDN_NET_CONNECTED)) { unsigned short hl; struct sk_buff *skb; + unsigned char *cpy_buf; /* * we need to reserve enough space in front of * sk_buff. old call to dev_alloc_skb only reserved @@ -869,11 +872,21 @@ isdn_ppp_write(int min, struct file *file, const char __user *buf, int count) return count; } skb_reserve(skb, hl); - if (copy_from_user(skb_put(skb, count), buf, count)) + cpy_buf = skb_put(skb, count); + if (copy_from_user(cpy_buf, buf, count)) { kfree_skb(skb); return -EFAULT; } + + /* + * Don't reset huptimer for + * LCP packets. (Echo requests). + */ + proto = PPP_PROTOCOL(cpy_buf); + if (proto != PPP_LCP) + lp->huptimer = 0; + if (is->debug & 0x40) { printk(KERN_DEBUG "ppp xmit: len %d\n", (int) skb->len); isdn_ppp_frame_log("xmit", skb->data, skb->len, 32, is->unit, lp->ppp_slot); -- 2.7.4
