[Xen-changelog] [xen stable-4.12] x86+Arm32: make find_next_{, zero_}bit() have well defined behavior

[patchbot@xxxxxxx]

commit 0e3fd5d2a16f0c4a3b67d1cb7038154ada0dfec1
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Dec 11 15:10:20 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Dec 11 15:10:20 2019 +0100

    x86+Arm32: make find_next_{,zero_}bit() have well defined behavior
    
    These functions getting used with the 2nd and 3rd arguments being equal
    wasn't well defined: Arm64 reliably returns the value of the 2nd
    argument in this case, while on x86 for bitmaps up to 64 bits wide the
    return value was undefined (due to the undefined behavior of a shift of
    a value by the number of bits it's wide) when the incoming value was 64.
    On Arm32 an actual out of bounds access would happen when the
    size/offset value is a multiple of 32; if this access doesn't fault, the
    return value would have been sufficiently correct afaict.
    
    Make the functions consistently tolerate the last two arguments being
    equal (and in fact the 3rd argument being greater or equal to the 2nd),
    in favor of finding and fixing all the use sites that violate the
    original more strict assumption.
    
    This is XSA-307.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Julien Grall <julien@xxxxxxx>
    master commit: 7442006b9f0940fb36f1f8470a416ec836e0d2ce
    master date: 2019-12-11 14:06:18 +0100
---
 xen/arch/arm/arm32/lib/findbit.S | 16 ++++++++--------
 xen/include/asm-x86/bitops.h     |  4 ++--
 2 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/xen/arch/arm/arm32/lib/findbit.S b/xen/arch/arm/arm32/lib/findbit.S
index b4e219a8d7..00674a31ef 100644
--- a/xen/arch/arm/arm32/lib/findbit.S
+++ b/xen/arch/arm/arm32/lib/findbit.S
@@ -42,8 +42,8 @@ ENDPROC(_find_first_zero_bit_le)
  * Prototype: int find_next_zero_bit(void *addr, unsigned int maxbit, int 
offset)
  */
 ENTRY(_find_next_zero_bit_le)
-               teq     r1, #0
-               beq     3b
+               cmp     r1, r2
+               bls     3b
                ands    ip, r2, #7
                beq     1b                      @ If new byte, goto old routine
  ARM(          ldrb    r3, [r0, r2, lsr #3]    )
@@ -83,8 +83,8 @@ ENDPROC(_find_first_bit_le)
  * Prototype: int find_next_zero_bit(void *addr, unsigned int maxbit, int 
offset)
  */
 ENTRY(_find_next_bit_le)
-               teq     r1, #0
-               beq     3b
+               cmp     r1, r2
+               bls     3b
                ands    ip, r2, #7
                beq     1b                      @ If new byte, goto old routine
  ARM(          ldrb    r3, [r0, r2, lsr #3]    )
@@ -117,8 +117,8 @@ ENTRY(_find_first_zero_bit_be)
 ENDPROC(_find_first_zero_bit_be)
 
 ENTRY(_find_next_zero_bit_be)
-               teq     r1, #0
-               beq     3b
+               cmp     r1, r2
+               bls     3b
                ands    ip, r2, #7
                beq     1b                      @ If new byte, goto old routine
                eor     r3, r2, #0x18           @ big endian byte ordering
@@ -151,8 +151,8 @@ ENTRY(_find_first_bit_be)
 ENDPROC(_find_first_bit_be)
 
 ENTRY(_find_next_bit_be)
-               teq     r1, #0
-               beq     3b
+               cmp     r1, r2
+               bls     3b
                ands    ip, r2, #7
                beq     1b                      @ If new byte, goto old routine
                eor     r3, r2, #0x18           @ big endian byte ordering
diff --git a/xen/include/asm-x86/bitops.h b/xen/include/asm-x86/bitops.h
index e66d861080..5a71afbc89 100644
--- a/xen/include/asm-x86/bitops.h
+++ b/xen/include/asm-x86/bitops.h
@@ -358,7 +358,7 @@ static always_inline unsigned int __scanbit(unsigned long 
val, unsigned int max)
     const unsigned long *a__ = (addr);                                      \
     unsigned int s__ = (size);                                              \
     unsigned int o__ = (off);                                               \
-    if ( __builtin_constant_p(size) && !s__ )                               \
+    if ( o__ >= s__ )                                                       \
         r__ = s__;                                                          \
     else if ( __builtin_constant_p(size) && s__ <= BITS_PER_LONG )          \
         r__ = o__ + __scanbit(*(const unsigned long *)(a__) >> o__, s__);   \
@@ -390,7 +390,7 @@ static always_inline unsigned int __scanbit(unsigned long 
val, unsigned int max)
     const unsigned long *a__ = (addr);                                      \
     unsigned int s__ = (size);                                              \
     unsigned int o__ = (off);                                               \
-    if ( __builtin_constant_p(size) && !s__ )                               \
+    if ( o__ >= s__ )                                                       \
         r__ = s__;                                                          \
     else if ( __builtin_constant_p(size) && s__ <= BITS_PER_LONG )          \
         r__ = o__ + __scanbit(~*(const unsigned long *)(a__) >> o__, s__);  \
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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