x86/entry/32: Use %ss segment where required

BugLink: https://bugs.launchpad.net/bugs/1854861
commit 4c4fd55d3d59a41ddfa6ecba7e76928921759f43 upstream.

When re-building the IRET frame we use %eax as an destination %esp,
make sure to then also match the segment for when there is a nonzero
SS base (ESPFIX).

[peterz: Changelog and minor edits]
Fixes: 3c88c692c287 ("x86/stackframe/32: Provide consistent pt_regs")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 58c5d078db90fc2c65b2eec6d1ba3b4771412d7f..05505f9e486c0ecd12e678e340b4349f185f89d7 100644 (file)
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -210,6 +210,8 @@
        /*
         * The high bits of the CS dword (__csh) are used for CS_FROM_*.
         * Clear them in case hardware didn't do this for us.
+        *
+        * Be careful: we may have nonzero SS base due to ESPFIX.
         */
        andl    $0x0000ffff, 3*4(%esp)
 
@@ -263,6 +265,13 @@
 .endm
 
 .macro IRET_FRAME
+       /*
+        * We're called with %ds, %es, %fs, and %gs from the interrupted
+        * frame, so we shouldn't use them.  Also, we may be in ESPFIX
+        * mode and therefore have a nonzero SS base and an offset ESP,
+        * so any attempt to access the stack needs to use SS.  (except for
+        * accesses through %esp, which automatically use SS.)
+        */
        testl $CS_FROM_KERNEL, 1*4(%esp)
        jz .Lfinished_frame_\@
 
@@ -276,20 +285,20 @@
        movl    5*4(%esp), %eax         # (modified) regs->sp
 
        movl    4*4(%esp), %ecx         # flags
-       movl    %ecx, -4(%eax)
+       movl    %ecx, %ss:-1*4(%eax)
 
        movl    3*4(%esp), %ecx         # cs
        andl    $0x0000ffff, %ecx
-       movl    %ecx, -8(%eax)
+       movl    %ecx, %ss:-2*4(%eax)
 
        movl    2*4(%esp), %ecx         # ip
-       movl    %ecx, -12(%eax)
+       movl    %ecx, %ss:-3*4(%eax)
 
        movl    1*4(%esp), %ecx         # eax
-       movl    %ecx, -16(%eax)
+       movl    %ecx, %ss:-4*4(%eax)
 
        popl    %ecx
-       lea     -16(%eax), %esp
+       lea     -4*4(%eax), %esp
        popl    %eax
 .Lfinished_frame_\@:
 .endm