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[PATCH v3 1/7] random: Simplify API for random address requests

From Jason Cooper <jason@lakedaemon.net>
Newsgroups linux.kernel
Subject [PATCH v3 1/7] random: Simplify API for random address requests
Date 2016-08-04 01:50 +0200
Message-ID <s2e5b-U8-17@gated-at.bofh.it> (permalink)
References <s012p-3rN-3@gated-at.bofh.it> <s2e5b-U8-3@gated-at.bofh.it>
Organization linux.* mail to news gateway

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To date, all callers of randomize_range() have set the length to 0, and
check for a zero return value.  For the current callers, the only way
to get zero returned is if end <= start.  Since they are all adding a
constant to the start address, this is unnecessary.

We can remove a bunch of needless checks by simplifying the API to do
just what everyone wants, return an address between [start, start +
range).

While we're here, s/get_random_int/get_random_long/.  No current call
site is adversely affected by get_random_int(), since all current range
requests are < UINT_MAX.  However, we should match caller expectations
to avoid coming up short (ha!) in the future.

All current callers to randomize_range() chose to use the start address
if randomize_range() failed.  Therefore, we simplify things by just
returning the start address on error.

randomize_range() will be removed once all callers have been converted
over to randomize_addr().

Signed-off-by: Jason Cooper <jason@lakedaemon.net>
---
Changes from v2:
 - s/randomize_addr/randomize_page/ (Kees Cook)
 - PAGE_ALIGN(start) if it wasn't (Kees Cook, Michael Ellerman)

 drivers/char/random.c  | 33 +++++++++++++++++++++++++++++++++
 include/linux/random.h |  1 +
 2 files changed, 34 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 0158d3bff7e5..61cb434e3bea 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1840,6 +1840,39 @@ randomize_range(unsigned long start, unsigned long end, unsigned long len)
 	return PAGE_ALIGN(get_random_int() % range + start);
 }
 
+/**
+ * randomize_page - Generate a random, page aligned address
+ * @start:	The smallest acceptable address the caller will take.
+ * @range:	The size of the area, starting at @start, within which the
+ *		random address must fall.
+ *
+ * If @start + @range would overflow, @range is capped.
+ *
+ * NOTE: Historical use of randomize_range, which this replaces, presumed that
+ * @start was already page aligned.  We now align it regardless.
+ *
+ * Return: A page aligned address within [start, start + range).  On error,
+ * @start is returned.
+ */
+unsigned long
+randomize_page(unsigned long start, unsigned long range)
+{
+	if (!PAGE_ALIGNED(start)) {
+		range -= PAGE_ALIGN(start) - start;
+		start = PAGE_ALIGN(start);
+	}
+
+	if (start > ULONG_MAX - range)
+		range = ULONG_MAX - start;
+
+	range >>= PAGE_SHIFT;
+
+	if (range == 0)
+		return start;
+
+	return start + (get_random_long() % range << PAGE_SHIFT);
+}
+
 /* Interface for in-kernel drivers of true hardware RNGs.
  * Those devices may produce endless random bits and will be throttled
  * when our pool is full.
diff --git a/include/linux/random.h b/include/linux/random.h
index e47e533742b5..098fec690d65 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -35,6 +35,7 @@ extern const struct file_operations random_fops, urandom_fops;
 unsigned int get_random_int(void);
 unsigned long get_random_long(void);
 unsigned long randomize_range(unsigned long start, unsigned long end, unsigned long len);
+unsigned long randomize_page(unsigned long start, unsigned long range);
 
 u32 prandom_u32(void);
 void prandom_bytes(void *buf, size_t nbytes);
-- 
2.9.2

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[PATCH 0/7] char/random: Simplify random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-28 23:30 +0200
  [PATCH 1/7] random: Simplify API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-28 23:30 +0200
    Re: [PATCH 1/7] random: Simplify API for random address requests Yann Droneaud <ydroneaud@opteya.com> - 2016-07-29 11:00 +0200
      Re: [PATCH 1/7] random: Simplify API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-29 20:30 +0200
  [PATCH 5/7] tile: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-28 23:30 +0200
  [PATCH 7/7] random: Remove unused randomize_range() Jason Cooper <jason@lakedaemon.net> - 2016-07-28 23:30 +0200
  [PATCH 6/7] unicore32: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-28 23:30 +0200
  [PATCH 4/7] arm64: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-28 23:30 +0200
    Re: [PATCH 4/7] arm64: Use simpler API for random address requests Will Deacon <will.deacon@arm.com> - 2016-07-29 15:50 +0200
  [PATCH 3/7] ARM: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-28 23:30 +0200
  [PATCH v2 7/7] random: Remove unused randomize_range() Jason Cooper <jason@lakedaemon.net> - 2016-07-30 17:50 +0200
  [PATCH v2 4/7] arm64: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-30 17:50 +0200
  [PATCH v2 2/7] x86: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-30 17:50 +0200
  [PATCH v2 5/7] tile: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-30 17:50 +0200
  [PATCH v2 3/7] ARM: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-30 17:50 +0200
  [PATCH v2 1/7] random: Simplify API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-30 17:50 +0200
    Re: [PATCH v2 1/7] random: Simplify API for random address requests Kees Cook <keescook@chromium.org> - 2016-07-31 18:50 +0200
      Re: [PATCH v2 1/7] random: Simplify API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-31 23:00 +0200
        Re: [PATCH v2 1/7] random: Simplify API for random address requests Kees Cook <keescook@chromium.org> - 2016-08-01 21:50 +0200
          Re: [PATCH v2 1/7] random: Simplify API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-08-02 01:20 +0200
            Re: [kernel-hardening] Re: [PATCH v2 1/7] random: Simplify API for random address requests Michael Ellerman <mpe@ellerman.id.au> - 2016-08-02 05:40 +0200
              Re: [kernel-hardening] Re: [PATCH v2 1/7] random: Simplify API for  random address requests Jason Cooper <jason@lakedaemon.net> - 2016-08-03 20:50 +0200
  [PATCH v2 0/7] char/random: Simplify random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-30 17:50 +0200
    [PATCH v2 6/7] unicore32: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-07-30 17:50 +0200
  [PATCH v3 2/7] x86: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-08-04 01:50 +0200
  [PATCH v3 6/7] unicore32: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-08-04 01:50 +0200
  [PATCH v3 5/7] tile: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-08-04 01:50 +0200
  [PATCH v3 7/7] random: Remove unused randomize_range() Jason Cooper <jason@lakedaemon.net> - 2016-08-04 01:50 +0200
    Re: [PATCH v3 7/7] random: Remove unused randomize_range() Andrew Morton <akpm@linux-foundation.org> - 2016-08-04 01:50 +0200
      Re: [PATCH v3 7/7] random: Remove unused randomize_range() Jason Cooper <jason@lakedaemon.net> - 2016-08-04 02:20 +0200
  [PATCH v3 0/7] char/random: Simplify random address requests Jason Cooper <jason@lakedaemon.net> - 2016-08-04 01:50 +0200
    [PATCH v3 4/7] arm64: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-08-04 01:50 +0200
    [PATCH v3 1/7] random: Simplify API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-08-04 01:50 +0200
      Re: [PATCH v3 1/7] random: Simplify API for random address requests Yann Droneaud <ydroneaud@opteya.com> - 2016-08-04 14:50 +0200
    [PATCH v3 3/7] ARM: Use simpler API for random address requests Jason Cooper <jason@lakedaemon.net> - 2016-08-04 01:50 +0200
    Re: [PATCH v3 0/7] char/random: Simplify random address requests Kees Cook <keescook@chromium.org> - 2016-08-04 04:50 +0200

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