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Subject[RFC][PATCH][bugfix] more checks for negative f_pos handling v2
Considering several approaches, I think this is the simplest..
Any idea ?

Thanks,
-Kame
==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Now, rw_verify_area() checsk f_pos is negative or not. And if
negative, returns -EINVAL.

But, some special files as /dev/(k)mem and /proc/<pid>/mem etc..
has negative offsets. And we can't do any access via read/write
to the file(device).

This patch introduce a flag S_VERYBIG and allow negative file
offsets.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
drivers/char/mem.c | 23 +++++++++++++----------
fs/proc/base.c | 2 ++
fs/read_write.c | 20 ++++++++++++++++++--
include/linux/fs.h | 2 ++
4 files changed, 35 insertions(+), 12 deletions(-)

Index: mmotm-2.6.31-Sep14/fs/read_write.c
===================================================================
--- mmotm-2.6.31-Sep14.orig/fs/read_write.c
+++ mmotm-2.6.31-Sep14/fs/read_write.c
@@ -205,6 +205,19 @@ bad:
}
#endif

+static bool
+__negative_fpos_check(struct inode *inode, loff_t pos, size_t count)
+{
+ struct inode *inode;
+ loff_t isize;
+
+ if (pos + count < pos)
+ return -EOVERFLOW;
+ if (IS_VERYBIG(inode))
+ return 0;
+ return -EINVAL;
+}
+
/*
* rw_verify_area doesn't like huge counts. We limit
* them to something that fits in "int" so that others
@@ -222,8 +235,11 @@ int rw_verify_area(int read_write, struc
if (unlikely((ssize_t) count < 0))
return retval;
pos = *ppos;
- if (unlikely((pos < 0) || (loff_t) (pos + count) < 0))
- return retval;
+ if (unlikely((pos < 0) || (loff_t) (pos + count) < 0)) {
+ retval = __negative_fpos_check(inode, pos, count))
+ if (retval)
+ return retval;
+ }

if (unlikely(inode->i_flock && mandatory_lock(inode))) {
retval = locks_mandatory_area(
Index: mmotm-2.6.31-Sep14/include/linux/fs.h
===================================================================
--- mmotm-2.6.31-Sep14.orig/include/linux/fs.h
+++ mmotm-2.6.31-Sep14/include/linux/fs.h
@@ -231,6 +231,7 @@ struct inodes_stat_t {
#define S_NOCMTIME 128 /* Do not update file c/mtime */
#define S_SWAPFILE 256 /* Do not truncate: swapon got its bmaps */
#define S_PRIVATE 512 /* Inode is fs-internal */
+#define S_VERYBIG 1024 /* Allow file's loff_t can be negative */

/*
* Note that nosuid etc flags are inode-specific: setting some file-system
@@ -265,6 +266,7 @@ struct inodes_stat_t {
#define IS_NOCMTIME(inode) ((inode)->i_flags & S_NOCMTIME)
#define IS_SWAPFILE(inode) ((inode)->i_flags & S_SWAPFILE)
#define IS_PRIVATE(inode) ((inode)->i_flags & S_PRIVATE)
+#define IS_VERYBIG(inode) ((inode)->i_flags & S_VERYBIG)

/* the read-only stuff doesn't really belong here, but any other place is
probably as bad and I don't want to create yet another include file. */
Index: mmotm-2.6.31-Sep14/drivers/char/mem.c
===================================================================
--- mmotm-2.6.31-Sep14.orig/drivers/char/mem.c
+++ mmotm-2.6.31-Sep14/drivers/char/mem.c
@@ -825,22 +825,23 @@ static const struct memdev {
const char *name;
const struct file_operations *fops;
struct backing_dev_info *dev_info;
+ bool verybig;
} devlist[] = {
- [ 1] = { "mem", &mem_fops, &directly_mappable_cdev_bdi },
+ [1] = { "mem", &mem_fops, &directly_mappable_cdev_bdi, true },
#ifdef CONFIG_DEVKMEM
- [ 2] = { "kmem", &kmem_fops, &directly_mappable_cdev_bdi },
+ [2] = { "kmem", &kmem_fops, &directly_mappable_cdev_bdi, true },
#endif
- [ 3] = {"null", &null_fops, NULL },
+ [3] = {"null", &null_fops, NULL, false },
#ifdef CONFIG_DEVPORT
- [ 4] = { "port", &port_fops, NULL },
+ [4] = { "port", &port_fops, NULL, false },
#endif
- [ 5] = { "zero", &zero_fops, &zero_bdi },
- [ 6] = { "full", &full_fops, NULL },
- [ 7] = { "random", &random_fops, NULL },
- [ 9] = { "urandom", &urandom_fops, NULL },
- [11] = { "kmsg", &kmsg_fops, NULL },
+ [5] = { "zero", &zero_fops, &zero_bdi, false },
+ [6] = { "full", &full_fops, NULL, false },
+ [7] = { "random", &random_fops, NULL, false },
+ [9] = { "urandom", &urandom_fops, NULL, false },
+ [11] = { "kmsg", &kmsg_fops, NULL, false },
#ifdef CONFIG_CRASH_DUMP
- [12] = { "oldmem", &oldmem_fops, NULL },
+ [12] = { "oldmem", &oldmem_fops, NULL, false },
#endif
};

@@ -868,6 +869,8 @@ static int memory_open(struct inode *ino
ret = dev->fops->open(inode, filp);
else
ret = 0;
+ if (dev->verybig)
+ inode->i_flags |= S_VERYBIG;
out:
unlock_kernel();
return ret;
Index: mmotm-2.6.31-Sep14/fs/proc/base.c
===================================================================
--- mmotm-2.6.31-Sep14.orig/fs/proc/base.c
+++ mmotm-2.6.31-Sep14/fs/proc/base.c
@@ -779,6 +779,8 @@ static const struct file_operations proc
static int mem_open(struct inode* inode, struct file* file)
{
file->private_data = (void*)((long)current->self_exec_id);
+ /* this file is read only and we can catch out-pf-range */
+ inode->i_flags |= S_VERYBIG;
return 0;
}



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