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[Backport to server-21.1-branch] render: Avoid possible double-free in ProcRenderAddGlyphs()

Olivier Fourdan requested to merge ofourdan/xserver:server-21.1-render-fix-use-after-free-addglyph into server-21.1-branch

ProcRenderAddGlyphs() adds the glyph to the glyphset using AddGlyph() and then frees it using FreeGlyph() to decrease the reference count, after AddGlyph() has increased it.

AddGlyph() however may chose to reuse an existing glyph if it's already in the glyphSet, and free the glyph that was given, in which case the caller function, ProcRenderAddGlyphs() will call FreeGlyph() on an already freed glyph, as reported by ASan:

  READ of size 4 thread T0
    #0 in FreeGlyph xserver/render/glyph.c:252
    #1 in ProcRenderAddGlyphs xserver/render/render.c:1174
    #2 in Dispatch xserver/dix/dispatch.c:546
    #3 in dix_main xserver/dix/main.c:271
    #4 in main xserver/dix/stubmain.c:34
    #5 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #6 in __libc_start_main_impl ../csu/libc-start.c:360
    #7  (/usr/bin/Xwayland+0x44fe4)
  Address is located 0 bytes inside of 64-byte region
  freed by thread T0 here:
    #0 in __interceptor_free libsanitizer/asan/asan_malloc_linux.cpp:52
    #1 in _dixFreeObjectWithPrivates xserver/dix/privates.c:538
    #2 in AddGlyph xserver/render/glyph.c:295
    #3 in ProcRenderAddGlyphs xserver/render/render.c:1173
    #4 in Dispatch xserver/dix/dispatch.c:546
    #5 in dix_main xserver/dix/main.c:271
    #6 in main xserver/dix/stubmain.c:34
    #7 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
  previously allocated by thread T0 here:
    #0 in __interceptor_malloc libsanitizer/asan/asan_malloc_linux.cpp:69
    #1 in AllocateGlyph xserver/render/glyph.c:355
    #2 in ProcRenderAddGlyphs xserver/render/render.c:1085
    #3 in Dispatch xserver/dix/dispatch.c:546
    #4 in dix_main xserver/dix/main.c:271
    #5 in main xserver/dix/stubmain.c:34
    #6 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
  SUMMARY: AddressSanitizer: heap-use-after-free xserver/render/glyph.c:252 in FreeGlyph

To avoid that, make sure not to free the given glyph in AddGlyph().

v2: Simplify the test using the boolean returned from AddGlyph() (Michel)
v3: Simplify even more by not freeing the glyph in AddGlyph() (Peter)

Fixes: bdca6c3d - render: fix refcounting of glyphs during ProcRenderAddGlyphs
Closes: #1659 (closed)
Signed-off-by: Olivier Fourdan ofourdan@redhat.com
Part-of: !1476 (merged)
(cherry picked from commit 337d8d48)

/cc @p12tic @alanc

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